Unsupported iridium (Ir) nanoparticles, that serve as standard oxygen evolution reaction (OER) catalysts in acidic electrolyzers, were investigated for electrochemical performance and durability in rotating disk electrode (RDE) half-cells. Fixed potential holds and potential cycling were applied to probe the durability of Ir nanoparticles, and performance losses were found to be driven by particle growth (coarsening) at moderate potential (1.4 to 1.6V) and Ir dissolution at higher potential (>= 1.8V). Several different commercially available samples were evaluated and standardized conditions for performance comparison are reported. The electrocatalyst RDE results have also been compared to results obtained for performance and durability in electrolysis cells. (C) The Author(s) 2016. Published by ECS. All rights reserved.