Reducing precious metal content and improving the efficiency of proton exchange membrane water electrolyzers is critical for producing renewable hydrogen cost-effectively. Mixed metal iridium oxide catalysts (AIrxOy, A = nonprecious metal) have demonstrated superior oxygen evolution reaction (OER) activity relative to IrO2 catalysts while utilizing less Ir. However, improved stability is required if these materials are to be implemented commercially. In this work, we use a combination of ex situ and in situ characterization techniques to study physical and electronic properties of Y2Ir2O7 as it evolves during OER in acidic electrolyte. We identify and quantify dissolution of Y and Ir, finding that this material exhibits similar stability to other reported mixed metal Ir oxides (104−105 molO2 evolved/molIr dissolved) and appears to become more stable over time. We find that the catalyst surface becomes enriched with Ir after electrochemical testing. We further monitored the Ir oxidation state in situ using high-energy resolution fluorescence detected X-ray absorption spectroscopy. Our results suggest that the Ir oxidation state is dynamic: an IrOx surface forms that is more oxidized than the bulk pyrochlore material but subsequently dissolves. Such detailed characterization of material properties can be used to develop design principles for improving catalyst stability.