Notwithstanding the possibility to amend the claims before the IB under Art. 19, an international application may be amended during the PCT Chapter II procedure. There are a number of important aspects to consider.

Firstly, the amendments filed must be such that they can be taken into consideration by the EPO in its capacity as IPEA. The conditions governing timing and formal aspects are explained in GL/PCT‑EPO H‑I, 2 to GL/PCT‑EPO H‑I, 6.

Any change in the claims, the description or the drawings, other than a rectification of obvious mistakes under Rule 91, a correction under Rule 26 or the furnishing of a missing partparts under Rule 20.5 or correct elements or parts under Rule 20.5bis, is considered an amendment. Unless withdrawn or superseded by later amendments, any change considered an amendment must be taken into consideration for the purpose of the international preliminary examination.

Secondly, amendments must be allowable, which means that they must not:

add to the application subject-matter which was not disclosed in the application as originally filed 
introduce other deficiencies (such as lack of clarity in the claims). 

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