In inter partes proceedings such as opposition proceedings the right to be heard is inextricably linked to the principle of equal treatment: no party should be given preferential treatment in the number of times it is allowed to present its case orally or in writing. The opposition division is therefore required to ensure that the parties can exchange their submissions in full and have equal opportunity to comment on them. If the opposition division considers a multiple exchange of submissions expedient, it must give each party equal opportunity to comment. Thus, the opposition division may invite a party to submit a rejoinder to the reply to the notice of opposition, but in that case it is obliged to invite the other party to respond to that rejoinder, after which it must decide again whether a third exchange of submissions is expedient. The fact that R. 79(3) EPC (R. 57(3) EPC 1973) leaves it to the opposition division ("if it considers it expedient") to invite the opponents to reply cannot be interpreted as an authorisation to derogate from the fundamental principle of equal treatment (see T 190/90, T 682/89 and T 439/91). According to T 669/90, (OJ 1992, 739), if the opposition division misleads a party into believing that it is not necessary to defend its interests by filing observations in reply to new facts and evidence filed by an adverse party, and if such new facts and evidence then form the basis for a decision adversely affecting the misled party, the latter has not had "an opportunity to present its comments" within the meaning of Art. 113(1) EPC. Such a procedure is also not a fair procedure and is contrary to the principle of good faith governing relations between the EPO and parties to proceedings before it (cf. T 532/91, T 678/06). Such inequality of treatment constitutes a substantial procedural violation.

In addition to the cases that follow, see in this chapter IV.C.3.4.6 "Opportunity to comment on new grounds for opposition".