The effect of halide ions on the electrochemical and corrosion behaviour of mild steel in sulphuric acid solution with and without some selected thiols, viz. 2-mercaptobenzoxazole, 2-mercaptobenzothiazole and 2-mercaptobenzimidazole, has been studied. Each of the halide ions and the investigated thiols were found to inhibit the corrosion process of mild steel when added separately to the corrosive medium, while the inhibition efficiency of thiols is higher than that of halides. It was found that the presence of halide ions together with the different thiols in the corrosive medium decreases the inhibition effect, specially at low concentrations. The surface coverage values which are obtained both from weight loss and polarization studies are approximately the same. The decrease in corrosion rate associated with an increase in the cathodic overvoltage and a positive shift in the corrosion potential, in the presence of the investigated thiols, denotes inhibition of a mixed type (predominantly anodic). Synergism of the investigated thiols and halides is discussed from the viewpoint of a model of co-adsorption of halide ions and thiols. Adsorption of the inhibitor was found to follow the Langmuir isotherm.