Anodic CO poisoning of a PEMFC was analysed by nonlinear frequency response analysis (NFRA) in a differential H(2)/H(2) cell. This special experimental setup excluded potential masking effects, emphasised the main mechanism of CO poisoning and made a simplified modelling approach possible. The main features of CO poisoning were investigated by means of steady state polarisation, EIS and NFRA. The main characteristics of CO poisoning in the NFRA spectra can be used as a "fingerprint" for diagnostic purposes.