The impedance characteristics of the Ni-YSZ cermet anode/YSZ interface were studied as a function of gas phase composition, temperature and anodic overpotential in methane-fuelled internal reforming YSZ fuel cells. AC impedance measurements carried out under open-circuit conditions in the temperature range 800-900 degreesC showed the presence of at least two rate-limiting processes which were influenced by temperature and gas phase composition, The apparent polarization resistance, R-app, of the Ni-YSZ cermet anode,YSZ interface was found to increase with increasing methane partial pressure and decreasing H2O partial pressure. The apparent capacitance, C-app, calculated from AC impedance data under open-circuit conditions was found to decrease with increasing methane partial pressure. AC impedance measurements carried out under closed-circuit conditions at 900 degreesC and four different P-H2O showed the appearance of a low-firequency inductive loop at high anodic overpotentials, in addition to the features observed under open-circuit conditions. The effect of gas phase composition on R-app was similar to the one observed under open-circuit conditions. The present results are compared with results presented in literature and discussed in the frame of the mechanism of the electrochemical and catalytic reactions taking place over the anode.