In T 142/97 the board held that the opposition division had violated the opponent's right to be heard under Art. 113(1) EPC 1973 by not hearing the witness offered in connection with a prior use that had been adequately substantiated in the notice of opposition. See also T 959/00 in which the board held that the opposition division's failure to hear the witness, and the absence in the decision under appeal of any reference to a reason why it had not been necessary to hear the witness, amounted to a fundamental procedural violation of the right to be heard.

In T 269/00 the board held that the case before it differed from T 142/97 because the ground of prior use was not adequately substantiated during the opposition period, but completed piecemeal during the opposition proceedings. The board concluded that not hearing the witness did not amount to a substantial procedural violation.

In T 474/04 (OJ 2006, 129), the board held that if assertions made in an unsworn witness declaration remained contested, a request to hear the witness had to be granted before these assertions were made the basis of a decision against the contesting party.

In T 909/03 the hearing of a witness took place in the morning and the oral proceedings continued in the afternoon. The board held that it was not necessary for a party to be given a copy of the minuted testimony before commenting on that testimony. During the oral proceedings the party had been given sufficient opportunity to comment. No substantial procedural violation had thus occurred.

In T 716/06 the board confirmed that where oral evidence of a witness was requested by a party the competent department should grant this request only if it considered this oral evidence necessary to clarify matters that were decisive for the decision to be taken. However, where an opponent requested that a witness be heard on an alleged public prior use and on the disclosure of a certain feature by this prior use, the competent department as a rule had to grant this request before deciding that the alleged public prior use was neither established nor constituted a novelty-destroying state of the art. See also T 1100/07.