According to Art. 115, second sentence, EPC a third party does not become a party to the proceedings. In principle, therefore, third parties have none of the procedural rights associated with such status, in particular the right to appeal and the right to be heard (T 1756/11). Their observations are to be examined as submitted and assessed with regard to credibility (T 951/93, T 1196/08).

In T 951/91 the board found that by interpreting Art. 115 EPC 1973 in the light of their object and purpose, it was clear that they were intended exclusively to cut down, and not to extend, the rights of third parties, still less to extend their rights beyond the rights of parties to the EPO (see also T 1756/11, T 1528/13).

According to T 390/07 the admissibility of third-party observations is entirely a matter for the board as a third party within the meaning of Art. 115 EPC is not a party to the proceedings and has no more than an opportunity to "present observations". While it is well-established by case-law that third‑party observations can be considered, both at first instance and on appeal, there is no obligation on the board beyond such consideration and no right of a third party to be heard on the admissibility of its observations or any evidence in support of them. The actual parties to proceedings naturally have the right to be heard in relation to such observations, but their admissibility is entirely a matter for the board. Parties to the proceedings can comment at any stage on new facts and evidence emerging from third-party observations filed after expiry of the opposition period, if they think that these could influence the decision. The opposition division or the board of appeal, exercising its discretion, is then obliged to rule on whether or not to admit the late-filed comments into the proceedings (T 1756/11).

In T 283/02, the opposition division had duly forwarded the observations by third parties to the patentee, who had made no comment. The failure of the opposition division to mention these observations in its decision did not constitute a procedural violation, even though a mention would have been desirable.