Polymer electrolyte membrane water electrolyser suffers mainly from slow kinetics regarding oxygen evolution reaction (OER). Noble metal oxides, like IrO2, RuO2, are generally more active for OER than metal electrodes, exhibiting low anodic overpotentials and high catalytic activity. However, issues like electrocatalyst stability under continuous operation and cost minimization through reduction of the catalyst loading are of great importance for the research community. In the present study, unsupported IrO2 of various particle sizes (different calcination temperatures) were evaluated for the OER and as anode electrodes for PEM water electrolyzers [1]. The elec-trocatalysts were synthesized by the modified Adams method [2][3] and the effect of calcination temperature on the properties of IrO2 electrocatalysts is investigated. Physicochemical charac-terization was conducted using X-ray Diffraction, BET, High-Resolution TEM and XPS analyses. For the electrochemical performance of synthesized electrocatalysts in the OER, cyclic voltammetry and linear sweep voltammetry were conducted in a typical 3-cell electrode configuration, using glassy carbon as working electrode, where the synthesized electrocatalysts were casted on, in 0.5 M H2SO4 solution. The materials, as anode PEM water electrolysis electrodes, were further evaluated in a typical electrolytic cell using Nafion®115 membrane as electrolyte and Pt/C as cathode elec-trocatalyst. The IrO2 electrocatalyst calcined at 400°C shows high crystallinity with 1.24 nm particle size, high specific surface area (185 m2 g-1) and a high OER activity of 177 mA cm-2 at 1.8 V as anode electrocatalyst for PEM water electrolyzer.