Art. 113(1) EPC, in which the requirement that the parties be heard is enshrined, provides that decisions of the EPO may only be based on grounds or evidence on which the parties have had an opportunity to present their comments. The word "grounds" is interpreted as referring to the essential reasoning, both legal and factual (see e.g. T 951/92, OJ 1996, 53; T 433/93, OJ 1997, 509). See also in this chapter IV.C.6.5. Opportunity to present comments" below.

Thus if an opposition division wishes to introduce a new ground of opposition into the proceedings in addition to the ground(s) substantiated in the notice of opposition, either of its own motion or upon request by an opponent, the patent proprietor must be informed (normally in writing) not only of the new ground of opposition (i.e. the new legal basis for the opposition), but also of the essential legal and factual reasons which could lead to a finding of invalidity and revocation. Thereafter the patent proprietor must have a proper opportunity to present comments in reply to the new ground and its substantiation (T 433/93, OJ 1997, 509, see also T 1283/11).

In T 656/96, the opponent had based his opposition solely on the invention's purported lack of inventive step. However, lack of novelty was advanced as a ground for opposition for the first time in the oral proceedings and in the absence of the patent proprietor, who had previously informed the division that he would not be attending oral proceedings. The opposition division revoked the contested patent, even though it had not originally raised a novelty objection in its communication under Art. 101(2) EPC 1973 (Art. 101(1) EPC). In such circumstances, the board referred to the case law of the Enlarged Board of Appeal distinguishing between the opposition ground of lack of novelty and that of lack of inventive step (G 7/95, OJ 1996, 626) and recognising the opposition division's competence to consider, exceptionally under Art. 114(1) EPC, fresh grounds for opposition which, prima facie, appear, either in whole or in part, to prejudice maintenance of the European patent (G 10/91, OJ 1993, 420). In the case in hand, the board held that, particularly in the light of the opinion on novelty expressed in the communication from the opposition division, no prima facie case against maintenance of the patent could be gathered from the file, so that the appellant had been entitled to consider that the question of novelty would play no part before the opposition division. The appellant must therefore have been taken by surprise by the decision taken in his absence by the opposition division at the oral proceedings to revoke the patent on the grounds of lack of novelty, grounds on which he had not been asked, nor had any reason, to express an opinion.

In T 270/94, the board found that the opposition division had infringed Art. 113(1) EPC by preventing opponent 1, who had based its opposition solely on lack of novelty, from commenting on an opposition ground of lack of inventive step duly submitted by opponent 2 and communicated to all the parties in accordance with R. 57(2) EPC 1973 (R. 79(2) EPC). Furthermore, the board observed that Art. 114(2) EPC, relied on by the opposition division, was not applicable to the case, because it indicated only that the EPO could disregard late-filed facts or evidence, whereas the case in hand involved arguments put forward by one of the parties to the proceedings in relation to a ground for opposition submitted in due time by another party.

In T 1164/00 the board held that the opposition division was in principle entitled to introduce a new ground for opposition even at the late stage of oral proceedings, since, in its view, it was apparent that the patent in suit did not comply with the requirements of Art. 83 EPC. However, in this case, the annex to the summons to oral proceedings did not contain any indication of the opposition division's intention to introduce a new ground for opposition, which meant that the appellant was made aware of the factual and legal reasons supporting the introduction of this new ground for the first time during the oral proceedings. The board therefore concluded that the appellant had not been notified in writing as early as possible and, not having been informed beforehand of the factual and legal reasons, had been taken by surprise and had not had a fair opportunity during the oral proceedings to prepare a proper defence against this new objection. In those circumstances, the opposition division ought to have adjourned the oral proceedings in order to give the appellant sufficient time, in accordance with Art. 113 EPC, to prepare a suitable defence against the new objections (see also T 433/93, OJ 1997, 509; T 817/93; T 1083/01; T 64/03 and T 27/14).

In T 2362/08, the board had to determine whether the patent proprietors' right to be heard according to Art. 113(1) EPC had been safeguarded with regard to the decision of the opposition division on insufficiency of disclosure. While the ground of insufficiency appeared in the notice of opposition, the objections of the opponents were merely based on two lines of argument. No further submission on insufficiency was filed by the opponents in writing and, before the oral proceedings took place, no further issue was raised by the opposition division. Specifically, in its communication accompanying the invitation to oral proceedings, the opposition division did not agree with the objections of the opponents, placed the burden of proof on them to establish that the skilled person would be unable to carry out the invention and noted that the opponents had not submitted any experimental evidence. In its decision on insufficiency, however, the opposition division developed a completely different line of argument on a substantive issue of insufficiency which was raised for the first time at the oral proceedings. Furthermore, the patent proprietors had to face an unexpected reversal of opinion from the opposition division, which discharged the opponents of the burden of proof, thereby shifting it to the patent proprietors. The board could not consider the simple formal hearing of the patent proprietors on the issue at the oral proceedings to have been a proper opportunity for them to present their comments. By presenting the new legal and factual framework only at the oral proceedings and by coming to a final decision on the issue without first having given the patent proprietors a proper opportunity to respond, the opposition division did not respect their right to be heard.