The inhibitive effect of 2-cyano-3-hydroxy-4(Ar) -5- anilino thiophene derivatives on the corrosion of 304 SS in 3 M HCl solution has been investigated by weight loss, galvanostatic polarization and potentiodynamic anodic polarization techniques. The results indicate that these compounds act as inhibitors retarding the anodic and cathodic corrosion reactions. The presence of inhibitors does not change the mechanism of either hydrogen evolution reaction or SS dissolution. Activation energy and some thermodynamic parameters were calculated and discussed. These compounds are mixed type inhibitors in the acid solution, and their adsorption on SS surface is found to obey the Temkin adsorption isotherm. The experimental results suggested that the % inhibition of these thiophene derivatives increases with the increase in inhibitor concentration and decreases with the increase in temperature of the medium. The synergistic parameter (S) was calculated and it was found that its value is greater than unity, which indicates that the enhanced inhibition efficiency caused by the addition of I-, SCN- and Br- is only due to synergistic effect. Relationship between molecular structure and their inhibition efficiency was elucidated by quantum chemical calculations using the semi-empirical self consistentmethods (SCF).