Once a party has submitted conclusive proof of its allegations, it has complied with the requirement of the burden of proof. The evidence need not prove the facts with absolute certainty in order to be deemed conclusive; it suffices that it proves that they are highly probable. If a party has discharged its burden of proof, the counterparty seeking to refute the conclusively established facts by way of counter-arguments bears the burden of proving the alleged facts (T 1162/07; see also T 270/90, OJ 1993, 725). In T 109/91 the board held that the burden of proof might shift constantly as a function of the weight of the evidence (confirmed e.g. in T 525/90, T 239/92 and T 838/92).
In ex parte proceedings, when the applicant challenges prima facie evidence concerning a fact, i.e. the nominal publication date of a document, and submits evidence to displace such prima facie evidence, the burden of proof shifts to the examining division to establish that the document was "made available to the public" within the meaning of Art. 54(2) EPC on that date (see T 929/94 with reference to T 750/94, OJ 1998, 32). According to T 128/87 (OJ 1989, 406), a party presenting a cheque to the EPO bore the burden of proof for its receipt by the EPO. However, if the party furnished sufficient proof that a certain document had been filed, such evidence shifted to the EPO the burden of providing a greater weight of evidence to the contrary (T 770/91 and J 20/85, OJ 1987, 102).
In opposition proceedings the burden of proving that the objections raised under Art. 100 EPC have been substantiated lies with the opponent. Where the opposition division has decided to maintain the patent and the opponent has filed an appeal against this decision, the burden of demonstrating on appeal that the reasons for maintaining the patent were justified is not automatically shifted to the patent proprietor (T 667/94). In other words, appeal proceedings do not result in a shift in the burden of proof to the detriment of the patentee; before the opposition division it is the opponent who bears the burden of proof of demonstrating that the patent does not fulfil the requirements of the EPC (T 1210/05). However, once the opposition division has decided to revoke the patent, the burden shifts to the proprietor of the patent, who then has to demonstrate on appeal that the reasons for revoking the patent were not sound, i.e. that the opposition division's decision was wrong as to the merits (T 585/92, OJ 1996, 129; the abbreviated version published in the OJ does not include point 3.2 of the Reasons which is relevant here).