A nanosized IrO2 anode electrocatalyst was prepared by a sulfite-complex route for application in a proton exchange membrane (PEM) water electrolyzer. The physicochemical properties of the IrO2 catalyst were studied by termogravimetryedifferential scanning calorimetry (TGeDSC), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The electrochemical activity of this catalyst for oxygen evolution was investigated in a single cell PEM electrolyzer consisting of a Pt/C cathode and a Nafion membrane. A current density of 1.26 A cm-2 was obtained at 1.8 V and a stable behavior during steady-state operation at 80 °C was recorded. The Tafel plots for the overall electrochemical process indicated a slope of about 80 mVdec-1 in a temperature range from 25 °C to 80 °C. The kinetic and ohmic activation energies for the electrochemical process were 70.46 kJ mol-1 and 13.45 kJ mol-1, respectively. A short stack (3 cells of 100 cm2 geometrical area) PEM electrolyzer was investigated by linear voltammetry, impedance spectroscopy and chrono-amperometric measurements. The amount of H2 produced was 80 l h-1 at 60 A under 330W of applied electrical power. The stack electrical efficiency at 60 A and 75 °C was 70% and 81% with respect to the low and high heating value of hydrogen, respectively.