The adsorption of (4-amino-2-methyl-5-pyrimidinyl methylthio) acetic acid (AMMPTA) on mild steel from hydrochloric acid solution was studied using the weight loss and hydrogen evolution techniques. The corrosion rates of mild steel in hydrochloric acid solution containing AMMPTA are measured as a function of AMMPTA concentration. The degree of surface covered is used to calculate the free energy of adsorption, DeltaG(ad) of AMMPTA. The variation of negative free energy of adsorption, DeltaG(ad) with surface coverage, theta is interpreted in terms of change in mechanism of inhibitor action of AMMPTA with concentration. Phenomenon of physical adsorption is proposed. The effect of AMMPTA is discussed from the viewpoint of adsorption model. The thermodynamic parameters of adsorption process were obtained from experimental data using Bockris-Swinkels adsorption isotherm. (C) 2003 Elsevier B.V. All rights reserved.