In this study, the influence of catalyst loading on the performance of a proton exchange membrane (PEM) water electrolyzer is investigated (Nafion 212 membrane; IrO2/TiO2 (anode) and Pt/C (cathode)). Due to the fast kinetics of the hydrogen evolution reaction (HER) on platinum (Pt), the Pt loading on the cathode can be reduced from 0.30 mg(Pt) cm(-2) to 0.025 mg(Pt) cm(-2) without any negative effect on performance. On the anode, the iridium (Ir) loading was varied between 0.20-5.41 mg(Ir) cm(-2) and an optimum in performance at operational current densities (>= 1 A cm(-2)) was found for 1-2 mg(Ir) cm(-2). At higher Ir loadings, the performance decreases at high current densities due to insufficient water transport through the catalyst layer whereas at Ir loadings < 0.5 mg(Ir) cm(-2) the catalyst layer becomes inhomogeneous, which leads to a lower electrochemically active area and catalyst utilization, resulting in a significant decrease of performance. To investigate the potential for a large-scale application of PEM water electrolysis, the Ir-specific power density (g(Ir) kW(-1)) for membrane electrode assemblies (MEAs) with different catalyst loadings is analyzed as a function of voltage efficiency, and the consequences regarding catalyst material requirements are discussed. (C) The Author(s) 2018. Published by ECS.