The corrosion inhibition of mild steel in 2 M hydrochloric acid solution by tributylamine has been investigated using the impedance technique. Tributylamine was studied in concentrations from 5 x 10(-4) M to 1 M at a temperature of 298 K. The inhibitor mechanism was treated as a substitutional adsorption process according to Flory-Huggins, Dhar-Flory-Huggins and Bockris-Swinkels isotherms. The best approach was obtained using the latter. A structural parameter, the projected molecular area of tributylamine, was calculated to elucidate inhibitor orientation in the adsorption process.