Powders of IrO2 were used as anode catalysts in water electrolysis cells with solid polymer electrolyte (SPE). The catalyst was prepared by a pyrolysis process in a nitrate melt at 340 degreesC and then annealed at different temperatures from 440 to 540 degreesC. The catalyst materials were applied to an electrode membrane assembly (MEA) and studied in situ in an electrolysis cell using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and stationary current density-potential relations. The impedance results clearly show that the non-annealed material possesses the highest electrocatalytic activity and that this activity deteriorates with increasing annealing temperature. The electrical conductivity, however, increases by increasing annealing temperature. Optimum annealing conditions were found at 490 C, where the total polarisation reaches a minimum in the high current density range (1-2 A cm(-2)), at the actual conditions. Increased crystallinity and size of the particles with increasing annealing temperature was observed by transmission electron microscopy (TEM). A very high electrochemical performance was obtained in a water electrolysis cell using IrO2 on the anode side and Pt on Vulcan XC-72 on the cathode side, corresponding to a current density of 1 A cm(-2) at 1.60 V. (C) 2003 Elsevier Ireland Ltd. All rights reserved.