The influence the composition of the cathode has on its structure and electrochemical performance was investigated for a Nafion content spanning from 10 to 70wt.%. The cathodes were formed on a Nafion membrane by the spray method and using 20wt.% Pt on Vulcan (E-TEK). Materials characterisation (SEM, STEM, gas and mercury porosimetry, electron conductivity) and electrochemical characterisation (steady-state polarisation curve, impedance spectroscopy in O2 and current-pulse measurements in N2) were performed. The impedance spectra were analysed using our dynamic agglomerate model. The results indicate that the agglomerate model is valid until a Nafion content of about 45wt.%. Pt/C and Nafion are homogeneously mixed for any composition and no Nafion film was observed. The cathodes containing 36–43wt.% Nafion display a single or double Tafel slope behaviour ascribed to diffusion limitations in the agglomerates. At larger Nafion content, the agglomerate model can describe the curves only by assuming a diffusion coefficient 3–4 decades smaller than that of gases. At such compositions, the porosity was only 10%. These results were interpreted as a blocking of the pores and a non-percolating pore system for too large Nafion contents.