Ultra-low Pt-loaded (3-54 A mu g cm(- 2)) gas diffusion electrodes were prepared by a direct current unbalanced magnetron sputtering process. Pt films were deposited directly onto the microporous layer of the gas diffusion electrode and tested in a half-cell and proton exchange membrane fuel cell. While the apparent activity towards oxygen reduction reaction under half-cell conditions is dependent on Pt loading, mass activity is Pt loading independent, indicating constant catalyst utilization. Assuming the validity of the porous electrode model, kinetic parameters are extracted from the experimental data. Unlike the results from the half-cell study, the U-I curves of the proton exchange membrane fuel cell at different loadings show a decrease of the catalyst utilization at higher Pt loadings. The Pt structure on the gas diffusion electrode was investigated by electron microscopy, atomic force microscopy and X-ray diffraction. [GRAPHICS] .