Department of Post-Graduate Studies and Research in Chemistry, Sahibganj College, Sahibganj-816 109, India Manuscript received 16 October 1998, revised 14 September 1999, accepted 23 September 1999 Inhibitive effect of various acid anions as corrosion inhibitor for mild steel in supply water has been studied. Molybdate ion is found to be an effective inhibitor under different operating conditions. The inhibitive effect is due to the blocking of the anodic sites of the metal by adsorption which follows Freundlich adsorption isotherm. Activation energy and other thermodynamic parameters for adsorption have been calculated.