The performance of composite electrodes as a function of the electrode composition is investigated experimentally through electrochemical impedance spectroscopy (EIS). In particular, two different composite cathodes are analysed, i.e. Pt/YSZ and LSM/YSZ. Typical impedance spectra have been obtained and used to evaluate the overall electrode polarisation resistance R-p. The qualitative features are similar for both cathodes in particular, in both cases the electrode performance (expressed in terms of R-p(-1)) shows a dome-shaped curve as a function of the electrode composition. Nevertheless. the quantitative aspects are very different: in particular. the maximum value of R-p(-1) is much higher for the LSM/YSZ electrode than for the Pt/YSZ one (22 vs. 1.5 S cm(-2)). Also, the composition which gives the maximum performance and the experimental values of the percolation thresholds are different. The reasons for the discrepancies are analysed and discussed.