A micro kinetic model of the Pt, O-2(g) was developed in state space form (model M3). The oxygen adsorption/desorption process was modeled as a precursor-mediated surface reaction. The surface diffusion of atomic oxygen and the electrochemical reduction of atomic oxygen near the three-phase boundary (tpb) were considered. It was shown that the simulated charge-transfer behavior of M3 is significantly different from models with ordinary Langmuir kinetics (model M2). The electrochemical rate constant was estimated from selected experimental data as k(10) = (6.05+/-0.25).10(6) m(3) /(mol.s). From experimental results it was concluded that only one adsorbed oxygen species is relevant for the dynamic behavior. In porous Pt electrodes binary gas phase diffusion of oxygen in O-2/N-2 gas mixtures becomes relevant at oxygen partial pressures below similar to 10(-3) atm. The general procedure for state and parameter estimation can be well adopted for the investigation of further reaction mechanisms.