Under Art. 19(2), fifth sentence, EPC, the opposition division can enlarge its composition by adding a legally qualified member where it deems this to be necessary.

In T 990/06 the board held that the opposition division's composition should be clear and transparent at all crucial stages of the proceedings so that the parties could verify compliance with Art. 19(2) EPC 1973 and check that the division members were impartial in keeping with G 5/91 (OJ 1992, 617). It is essential that the opposition division's composition is lawful. Any doubt surrounding this casts doubt on the lawfulness of the decision. In the case in question, there were discrepancies between the documents issued to the parties and an internal instruction. Although the division had apparently been first enlarged and then reduced again in size, there was no information about either of these measures on file, so the board was unable to verify whether the decision given at the oral proceedings had been made by a lawfully composed opposition division and whether the written reasons for the decision had been issued by the same panel. The procedure for enlargement of the opposition division by addition of a legally qualified member under Art. 19(2) EPC 1973 – or their later withdrawal – must be verifiable.

In T 1254/11 the board held that an opposition division enlarged to four members pursuant to Art. 19(2) EPC 1973 could in principle be reduced again to three members. It was for the four-person panel to decide on the reduction. In this respect the board concurred with T 990/06. In deciding on the reduction, the opposition division consisting of four members must properly exercise its discretion. The board assumed arguendo that the fact that neither a decision to enlarge nor a decision to reduce the opposition division had been added to the publicly available file and the fact that the appointment of the new chairman could only be traced from the internal register of the EPO both constituted fundamental deficiencies of the proceedings before the opposition division. However, unlike in T 990/06, it was possible to determine from the file that the division had been lawfully enlarged and, at a later stage, lawfully reduced again.

In T 1088/11 the board held that in principle, an opposition division may set aside a decision to enlarge its composition. However, where an opposition division has been enlarged according to Art. 19(2) EPC, but the case is nevertheless decided in a composition of three members, there should be clear evidence on the public file that a decision to set aside enlargement was taken by the opposition division in its four member composition prior to the final decision.