Understanding the deactivation mechanisms affecting the state-of-the-art, Ir oxide catalysts employed in polymer electrolyte water electrolyser (PEWE-) anodes is of utmost importance to guide catalyst design and improve PEWE-durability. With this motivation, we have tried to decouple the contributions of various degradation mechanisms to the overall performance losses observed in rotating disk electrode (RDE) tests on three different, commercial Ir oxide catalysts (pure or supported on Nb2O5). Specifically, we investigated whether these performance decays stem from an intrinsic deactivation of the catalysts caused by alterations in their oxidation state, crystalline structure, morphology and/or Ir-dissolution, and also assessed possible decreases in the catalyst loading caused by the delamination of the materials over the course of these OER-stability tests. Additionally, we also examined recently reported artifacts related to the use of RDE voltammetry for such measurements and found that neither these nor the above mechanisms (or combinations thereof) can cause the totality of the observed performance losses. Beyond these uncertainties, complementary PEWE-tests showed that this apparent RDE-instability is not reproduced in this application-relevant environment.