A quantitative approach to evaluate the effect of the passive film on the corrosion resistance of ferrous alloys is introduced. It is based on recent developments of the theoretical description of ionic and electronic transport through passive films and newly introduced electrochemical techniques. The procedure is tested on an extensive set of experimental data for the passive state of Fe-Cr-Mo alloys in neutral solutions obtained using a combination of electrochemical techniques. The fair agreement between theory and experiment can be taken as proof of the validity of the present approach and the assumptions involved. The model predictions are in agreement with the accelerating effect of Mo on the rate of transpassive dissolution of Cr from Fe-Cr-Mo alloys. The comparison of model calculations and experimental data also shows that the presence of Mo in the film slows down the rate of iron dissolution via the divalent interstitial route in the passive range. These impacts of Mo can be explained by its presence in both octahedral and tetrahedral positions in the oxide lattice. This kind of separate estimation of the impact of different alloying additives can thus lead to reliable predictions of the corrosion behaviour of alloys in a range of conditions.