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The influence of inhibitor concentration and temperature on the corrosion behaviour of steel in molar HCl solution has been investigated by weight loss method. Results obtained show that the inhibitory effect of 2-phenylthieno (3, 2-b) quinoxaline (P4) increases with increasing P4 concentration to attain the highest value (95%) at 5 10-4M. The determination of the corrosion rate at the temperature range (308-353K) indicates that E% decreases with rise of temperature. Arrhenius law and its transition equation lead to estimate activation parameters of corrosion process. P4 inhibits well steel at moderate temperature and adsorbs according to the Langmuir isotherm. Plots of logarithm of corrosion rate function of the reciprocate of temperature as well as Van’t hoff and Gibbs equations facilitated the access to thermodynamic parameters such as equilibrium constant, adsorption heat and adsorption entropy. The various parameters of activation and adsorption may be a good tool to discuss manner of adsorption of organic compound. Attempt to correlate molecular structure to quantum indices was made.