Overpotential and AC impedance spectra were measured to study the reaction model of dense Sm0.5Sr0.5CoO3 (SSC) as SOFC cathode. From the P-O2 dependence of interfacial conductivity, the rate determining step of dense SSC electrode was adsorption and desorption processes at the surface of the electrode. Rate constants of adsorption and desorption were calculated from the interfacial conductivity of the dense electrode to be 3 x 10(-5) mol cm(-2) s(-1) atm(-1) and 2 x 10(-8) mol cm(-2) s(-1), respectively. These were approximately one order of magnitude larger than the corresponding values calculated for La0.6Sr0.4CaO3. These rate constants can elucidate the overpotentials of porous electrodes.