Six new inhibitors were prepared by modification of epoxidized linseed oil and oleic acid with methyl-, ethyl- and butylamines. The material used for testing was mild steel and the environments were 0.05N HCl at 70°C or vapours evolving from gasoline-2N HCl mixtures. Efficiency values as high as 92% were obtained for some compounds, the corresponding values for two commercial inhibitors being 91 and 64%. It has been shown that the efficiency in the gaseous environment increased with branching of the inhibitor molecule and with increase in the length of the chain attached to the functioning group. In the liquid environment branching lowered the efficiency of the inhibitor. The results conformed with predictions based on Taft's constants and Langmuir isotherms.