Iridium oxide nanoparticles are prepared via a hydrothermal method, treated at different calcination temperatures, and their activities and stabilities for the oxygen evolution reaction (OER) evaluated. The catalysts are physicochemically characterized using several techniques including X-ray diffraction, energy dispersive X-ray spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Voltammetric profiles obtained for the catalysts calcined up to 300 degrees C are similar to that of electrochemically prepared hydrous iridium oxide, while the CV profiles are characteristic of thermally prepared iridium oxide after calcination at higher temperatures. Performance as an OER catalyst decreases with increasing IrOx calcination temperature, while the opposite trend in stability is observed for these materials. Catalysts calcined between 400 and 500 degrees C exhibit better balances between activity and stability. However, despite higher performance losses, the non-calcined IrOx material still exhibits higher mass activity at the end of the aging experiments at electrode potentials up to 1.6 V vs. RHE. The causes of electrode activity degradation are investigated using identical location transmission electron microscopy, which reveal that III). electrode instability is due to the degradation of the thin IrOx layer, in addition to iridium oxide dissolution. (C) 2017 Elsevier B.V. All rights reserved.