In the absence of relevant EPC provisions, the principles concerning the allocation of the burden of proof have been developed through the case law of the boards of appeal. Each party bears the burden of proof for the facts it alleges. At all events, a party cannot deny facts which related to its own actions or were the subject of its own perceptions on the ground of "ignorance", if the presentation of the facts in question is essentially the party's responsibility (T 247/98).

With regard to principles of procedural law generally recognised in the contracting states, a party relying on Art. 125 EPC and asserting that German law allows for the revision of a decision of a court of final jurisdiction in the case of a fundamental procedural violation, will also have to produce evidence that such a procedural principle exists in most EPC contracting states and is therefore "generally recognised" for the purpose of Art. 125 EPC 1973 (T 843/91 date: 1993-08-05, OJ 1994, 832). Following on from that, another board stated in T 833/94 that, when the evidence offered did not directly indicate the truth or falsity of the fact in issue, but required knowledge of the national law and patent practice, such law and practice had to be proved, as did any other fact on which a party based its arguments.

In T 1201/14 (transfer of right of priority), as regards the applicable rules for assessing the evidence adduced (affidavits), the board concluded the following: the burden of proving a valid transfer of the right of priority lay with the patent proprietor (appellant) since it was the one claiming that right (see e.g. T 1008/96, point 3.3 of the Reasons; T 1056/01, point 2.10 of the Reasons; T 493/06, point 8 of the Reasons; T 205/14, point 3.5 of the Reasons). This rule was all the more applicable taking into account that the patent proprietor (appellant) alone had access to the relevant evidence when it asserted a concluded transfer of rights.

In ex parte proceedings the applicant bears the burden of proof for the facts in his favour, e.g. the fact that a document cited by the examining division does not form part of the state of the art (T 160/92, OJ 1995, 35), the fact that the conditions laid down in Art. 123 EPC have been met (T 383/88) or the fact that a limitation of the claims is admissible (T 2/81, OJ 1982, 394). Sufficient disclosure also obliges the applicant to provide evidence of the skilled person's relevant knowledge if there is reason to believe it may not cover all the subject-matter claimed. If reasonable doubts exist about the scope or public nature of the specialist knowledge required for adequate disclosure of the invention, the applicant discharges his duty to disclose by submitting suitable evidence in support of the facts in his favour, in order to fulfil his duty to disclose the invention (T 82/07). In T 32/95, the board referred to a passage from the Guidelines (now G‑IV, 1 "State of the art" – November 2018 version, which is cited in full in connection with T 750/94, under chapter III.G.4.3.1 above).

As far as issues relating to patentability requirements are concerned, the burden of proof in examination proceedings cannot lie initially with the applicant. If an examining division raises an objection, it must be appropriately substantiated. In T 578/06 the examining division had failed to provide such substantiation. Also, the passage in the "Case Law of the Boards of Appeal of the EPO" (see paragraph immediately above) could not support its view that in ex parte proceedings the applicant has the burden of proof for facts in his favour, as it appeared to have been taken out of its context. In this respect, the board clarified that all the procedural situations referred to in this passage were those where, in response to a substantiated objection from the examining division, the applicant was required to support its contention.

It is not sufficient in opposition proceedings for the opponent to impugn a granted patent with an assertion which cannot be substantiated. And if the EPO is unable to establish the facts of its own motion, the proprietor is given the benefit of the doubt (T 219/83, OJ 1986, 211, 328; see also T 293/87 and T 459/87). According to T 200/94 (following established case law), if a board's investigation does not enable it to verify beyond reasonable doubt the facts alleged, this goes to the detriment of the party needing to prove them, i.e. the party who relies on these facts. In appeal proceedings, the party's status as opponent makes no difference when it comes to assigning the burden of proof (this follows also from T 740/90, T 270/90, OJ 1993, 725 and T 381/87, OJ 1990, 213). Parties to proceedings bear separate burdens of proving the facts they allege, and if a fact that is material to the decision is not proven, then the decision goes against the party that failed to discharge that burden. In T 1469/08, the appellant (opponent), who was alleging prior use, argued that all the prostheses in question, whatever their article code and batch of manufacture, had the same structure as the one submitted as evidence to the EPO, but failed to provide any evidence to support that (the board cited T 270/90, OJ 1993, 725, point 2.1 of the Reasons; T 355/97, point 2.5 of the Reasons).

In T 998/04 the board emphasised as an obiter dictum that the burden of proof for alleged lack of patentability lay with the opponent and could not be dispensed with by requesting the board to carry out its own investigations, in particular, by summoning four named witnesses, by commissioning an independent expert to carry out experimental tests and by allowing individuals from the appellant company to attend any tests and to question witnesses or experts. Moreover, granting these requests would not be consistent with the character of the post-grant opposition proceedings under the EPC which were in principle to be considered as contentious proceedings between parties normally representing opposite interests, who should be given equally fair treatment (see G 9/91, OJ 1993, 408, point 2 of the Reasons). If the opponent disputes the existence of novelty, an inventive step or the reproducibility of the invention, it bears the burden of proof in this respect at first and second instance (e.g. T 762/04; T 382/93; T 16/87, OJ 1992, 212; T 182/89, OJ 1991, 391). In case of uncertainty about a prior-art disclosure, the patentee should be given the benefit of the doubt (T 1003/96; see also T 230/92, T 345/86, T 601/91 and T 968/91).

A technical problem set out in a patent is considered to be credibly solved by a claimed invention if there are no reasons to assume the contrary. Under such circumstances, the burden is normally on the opponent to prove the opposite or at least provide evidence casting doubt on the alleged solution of the problem. If the opponent succeeds, the burden of proof shifts to the patent proprietor who must then prove its assertions (see e.g. T 1797/09). Where, however, the opponent succeeds in casting serious doubt on the persuasiveness of the patentee's evidence merely by arguing that this evidence was subject to erroneous evaluation by the examining division, this allegation may render the patentee's evidence inappropriate but cannot justify the conclusion that the invention fails to solve the existing technical problem. Such allegation does not discharge the opponent from the burden of submitting convincing counter-evidence that the claimed subject-matter cannot solve the technical problem (T 596/99).

The appellant relied upon experimental reports. However, these tests did not prove its allegations beyond all reasonable doubt. The proprietor (appellant) submitted that the burden of proof lay with the respondents (opponents) to show that the amendments did in fact extend the protection conferred (Art. 123(3) EPC). However, it was the proprietor (appellant), who had amended the patent as granted and who was responsible for demonstrating, beyond all reasonable doubt, that said amendment did not extend the protection conferred (T 2285/09).

Reversely, the patentee cannot relieve itself of the burden of providing counter‑evidence for facts it alleges by simply stating that all the evidence was within the power of its opponent (with reference to T 472/92, OJ 1998, 161;), without providing proof for the contention that it is not within its power to obtain counter-evidence (T 254/98). As far as apportioning the burden of proof is concerned, it is irrelevant whether or not the patentee can access the requisite evidence more easily than the opponent (T 1162/07; see also T 1710/12).