Abstract The film of corrosion products (CP) often has certain protective properties and leads to significant decrease in corrosion rate. In case of corrosion inhibition an inhibitor is adsorbed on a CP film, but corrosion slowing down is usually connected only with the presence of phase inhibitor layer on the metal surface without taking into account the protective CP film. Such assumption leads to serious exaggeration of inhibitor protective effect and the more protective CP film forms on metal the greater error of such estimation is. Thus, separate estimation of protective effects of CP and inhibitor films is of interest. This paper offers the modification of method for separate estimation of corrosion products and inhibitor films contributions to total corrosion protection. Modification 1 accounts thickness change of corrosion products film under the inhibitory influence. Modification 2 takes into account solution of corrosion product film in liquid and also thickness reduction of this film. Modification 3 considers additivity of corrosion protection coefficients of superficial films instead of additivity of corrosion protective effects. The comparison of modified methods results is carried out by the example of inhibition of steel hydrogen sulfide corrosion in a liquid phase by tributylamine.