Platinum and iridium mixtures or alloys are the state-of-the-art bifunctional oxygen catalysts for unitized regenerative fuel cells (URFCs). Recently, we have shown that the degradation of these catalysts represents a serious challenge. In this work we explore the degradation mechanisms further by considering different catalysts and experimental conditions. For this, Pt/IrOx with varying Pt to Ir ratios are synthesized and their performances are evaluated. It is found that catalysts with higher Pt and Ir contents have higher oxygen reduction (ORR) and evolution reaction (OER) activities, respectively. The materials stability is investigated under a potential pulses protocol. Using on-line dissolution analysis, it is found that, besides the transient Pt dissolution identified in the previous works, continuous dissolution of Ir during the OER must be considered. The latter is assigned to a continuous formation of common OER and Ir dissolution intermediates. Further, identical location microscopy is employed to reveal the changes in the catalyst layer structure and morphology. It is shown that dissolution triggers significant particles agglomeration/coalescence, which are the main causes for the catalysts degradation. These processes must be attenuated for a feasible long-term application of Pt/IrOx materials as bifunctional oxygen catalyst. (C) 2019 Elsevier Ltd. All rights reserved.