Art. 19(2), first and second sentence, EPC refers to the participation of members of the opposition division at every stage of the grant proceedings, not only to their involvement in the final decision (T 476/95). If only one examiner took part in the prior proceedings, that examiner may not be chairman in the subsequent opposition proceedings under Art. 19(2) EPC (T 939/91). It is established case law (T 251/88, T 939/91, T 382/92, T 476/95, T 838/02, T 825/08, T 1349/10, T 1700/10, T 234/11) that an improper composition of the opposition division is a substantial procedural violation.
In T 1788/14 the opposition division's decision had been taken in a composition that differed from the examining division only in that a new chairman had been appointed. The board regarded this to be a substantial procedural violation.
In T 382/92 the board held that in cases of a breach of Art. 19(2) EPC it follows from the lack of jurisdiction of the department of first instance that the impugned decision is void ab initio (see also J 38/97).