a b s t r a c t The paper presents an investigation on the electrocatalytic activity of co-sputtered PtIr thin films towards the ORR. The catalyst composition was controlled by varying the sputtering power of the Ir target in the range 20e100 W at constant power of 100 W for the Pt target. The films were analyzed applying SEM, XRD, EDX, XPS (physical characterization); stationary and quasi-stationary polarization curves, cyclic voltammetry and ac impedance ("in situ" tests performed in single PEMFC). The influence of the Pt/Ir ratio on the atomic interaction, morphology, and the resulting electrocatalytic activity was studied in detail. The contribution of the film content, morphology and the Pt/Ir electrocatalytic synergism to the electrode performance was evaluated. The optimal balance between the favourable effect of Ir addition and the lost of active surface sites due to substitution of Pt for Ir was established.