The influence of Mo addition (5-10 wt.%) on the transpassive dissolution, secondary passivation and oxygen evolution on a Fe +12%Cr alloy in 1M H2SO4 is studied by voltammetric and AC impedance measurements within the frame of an investigation of the impact of Mo on the stability of the passive state of ferrous alloys in acidic media. A semi-quantitative model of the processes is proposed, based on the experimental results obtained and model approaches presented in the recent literature. It was found to be consistent with the experimental data and could serve as a first approximation to a generalized model of transpassivity.