Art. 117(1)(g) EPC provides for sworn statements in writing as a means of evidence (rare in practice). However, since any kind of evidence is admissible in proceedings before the EPO, other, less solemn types of written statement (common in practice), e.g. statutory declarations, are also accepted. It is for the boards then to assess their probative value on a case-by-case basis. Such declarations consist of a witness's written statement, the main purpose of which is to avoid the need to hear that witness. The board may, however, decide to order such a hearing, for example if a party so requests. The terminology used in the case law includes "affidavits", "statutory declarations" and "unsworn statements" (French: "déclarations écrites", "déclarations sur l'honneur" and "attestations"; German: "eidesstattliche Versicherungen" and "eidesstattliche Erklärungen").

Sworn statements in writing, one of the means of giving evidence listed in Art. 117(1) EPC, are not automatically ordered simply at the request of one party. All the means of giving or obtaining evidence covered by Art. 117 EPC are subject to the discretion of the department concerned, which will order their use only if it considers this necessary (T 798/93, OJ 1997, 363).

Sworn statements (Art. 117(1)(g) EPC) are to be distinguished from "statutory declarations", which are not given on oath. Albeit not explicitly covered by Art. 117(1)(g) EPC, statutory declarations are regarded as admissible means of evidence and are taken into account in accordance with the principle of the unfettered consideration of evidence (see e.g. T 770/91, J 10/04 and T 535/08; cf. T 1127/97, declaration submitted not a "eidesstattliche Versicherung" under German law).

A statutory declaration ("eidesstattliche Erklärung" or "déclaration tenant lieu de serment") is a means of giving evidence within the meaning of Art. 117(1) EPC and as such is subject to the principle of free evaluation of evidence (T 558/95, cf. T 482/89, OJ 1992, 646, point 2.1 of the Reasons; T 575/94, point 3.7 of the Reasons). The board in T 443/93, which had French as the language of the proceedings, referred to a document entitled "eidesstattliche Erklärung" alternately as a "déclaration sous serment" (affidavit) and a "déclaration écrite" (written statement); T 563/02 referred to a document entitled "affidavit" as "déclaration sur l'honneur". The board in French-language case T 2338/13 explicitly referred to affidavits as "attestations". The board in T 474/04 (OJ 2006, 129), which had English as the language of the proceedings, referred to a document entitled "eidesstattliche Versicherung" as a "declaration in lieu of an oath" and an "unsworn witness declaration". In T 703/12, the board called a document of this kind entitled "eidesstattliche Versicherung" a "statutory declaration" and in T 1231/11 an "affidavit". In proceedings before the EPO even a simple declaration can be an admissible means of evidence within the meaning of Art. 117(1) EPC (T 474/04, OJ 2006, 129 – a decision establishing a number of points relating to the law of evidence). The EPO accepts unsworn solemn declarations the same way it accepts other unsworn statements (T 970/93, T 313/04).

In T 915/12 the board considered that there was not sufficient evidence to prove that D16 – an extract from the encyclopedia- was made available to the public before the priority date (5 February 2001). The printing (2000) and copyright (1999) years indicated in D16 could not alone prove accessibility before early 2001. The hand-written annotation made by Ms S. – Head of Collections Department University – on the cover page did not fulfil the requirements of form and content which usually apply for affidavits or similar documents.

In R 3/10 the Enlarged Board took signed declarations of persons who had attended the oral proceedings into account.