The proprietors of the patent generally have to indicate the basis in the original application documents or claims of the granted patent from which the amendments may be derived (Art. 100(c) and Art. 123(2)). In addition, they should file observations as regards the patentability of the subject-matter of the patent as amended (with reference to Art. 100(a) and Art. 100(b)), taking into account the state of the art and objections raised in the opposition notice together, where appropriate, with the evidence presented in support.

Opposition is not an opportunity to re-examine the whole patent; it is the amendments introduced into the patent which must be examined as to whether they comply with the EPC as a whole (see G 3/14, T 227/88 and T 301/87). Therefore the opposition division will check that the patent, by the amendments themselves, does not contravene the requirements of the EPC (with the exception of Art. 82, see G 1/91 and D‑V, 2). With respect to Art. 84, see D‑V, 5. For the form of amended documents, see H‑III, 2.2, to H‑III, 2.4. The formal requirements, in particular Rules 30 to Rule 34, and Rules 42, Rule 43, Rule 46, Rule 48 and Rule 50, must also be satisfied (see Rule 86).