Once a party has submitted conclusive proof of its allegations, it has complied with the requirement of the burden of proof. The evidence need not prove the facts with absolute certainty in order to be deemed conclusive; it suffices that it proves that they are highly probable. If a party has discharged its burden of proof, the counterparty seeking to refute the conclusively established facts by way of counter-arguments bears the burden of proving the alleged facts (T 1162/07; see also T 270/90, OJ 1993, 725). In T 109/91 the board held that the burden of proof might shift constantly as a function of the weight of the evidence, i.e. if a party provided enough evidence to demonstrate a fact to the conviction of the board, the mere allegation of the contrary by the other party is not convincing (confirmed e.g. in T 525/90, T 239/92 and T 838/92).

In ex parte proceedings, when the applicant challenges prima facie evidence concerning a fact, i.e. the nominal publication date of a document, and submits evidence to displace such prima facie evidence, the burden of proof shifts to the examining division to establish that the document was "made available to the public" within the meaning of Art. 54(2) EPC on that date (see T 929/94 with reference to T 750/94, OJ 1998, 32). According to T 128/87 date: 1988-06-03 (OJ 1989, 406), a party presenting a cheque to the EPO bore the burden of proof for its receipt by the EPO. However, if the party furnished sufficient proof that a certain document had been filed, such evidence shifted to the EPO the burden of providing a greater weight of evidence to the contrary (T 770/91 and J 20/85, OJ 1987, 102). The more recent case T 538/09 addressed this issue of burden of proof and standard of proof in its analysis of T 750/94 and T 151/99.

In ex parte case T 545/08 the board said that it is a general principle that, when the examining division raises objections, the burden of proof lies initially with it. This means that objections must be reasoned and substantiated, and must show that, on the balance of probabilities, the objection is well-founded (see Guidelines, G‑IV, 7.5.3 – November 2018 version). With respect to the publication date of a cited document, at least prima facie evidence is required. Prima facie evidence means evidence which is sufficient, on its own, to establish a fact or to raise a presumption of the truth of a fact unless controverted (see T 750/94, point 6 of the Reasons; T 526/12, point 1.4 of the Reasons; T 1066/13, "directory listing" – no prima facie evidence). Thus, not every indication or hint qualifies as prima facie evidence. If the objection is properly raised, it is then up to the applicant to prove otherwise or at least to submit evidence to displace the prima facie evidence. If the applicant successfully challenges prima facie evidence concerning a fact, e.g. the nominal publication date of a document, the burden of proof shifts back to the examining division to establish that the document was made available to the public (see e.g. T 929/94, point 2.1 of the Reasons). As to the case in hand in T 545/08, the board found, among other things that, in the absence of prima facie evidence of the public availability of document D1, the examining division had not been entitled to consider it as prior art in its first substantive communication without providing further explanations and evidence as to its public availability before the priority date (see also T 1961/13). Thus, the objection was not properly raised. It could not generate an obligation on the applicant's side to submit evidence against the assumed publication date.

In opposition proceedings the burden of proving that the objections raised under Art. 100 EPC have been substantiated lies with the opponent. Where the opposition division has decided to maintain the patent and the opponent has filed an appeal against this decision, the burden of demonstrating on appeal that the reasons for maintaining the patent were justified is not automatically shifted to the patent proprietor (T 667/94). In other words, appeal proceedings do not result in a shift in the burden of proof to the detriment of the patentee; before the opposition division it is the opponent who bears the burden of proof of demonstrating that the patent does not fulfil the requirements of the EPC (T 1210/05). However, once the opposition division has decided to revoke the patent, the burden shifts to the proprietor of the patent, who then has to demonstrate on appeal that the reasons for revoking the patent were not sound, i.e. that the opposition division's decision was wrong as to the merits (T 585/92, OJ 1996, 129; point 3.2 of the Reasons).