50 pp sodium salt of 1-hydroxyethane-1,1-diphosphonic acid (HEDP) offered only 11% inhibition efficiency (IE) to corrosion of mild steel immersed in neutral aqueous environment containing 60 ppm chloride. When 50 ppm is added to 50 ppm HEDP, the IE increased to 98%. The influence of Zn2+ on the corrosion-inhibition by HEDP is discussed based on the results obtained from UV-visible, FTIR and luminescence spectra. Presence of Zn2+ facilitates the transport of HEDP from the bulk of the solution to the metal surface; both the anodic reaction and cathodic reaction are controlled effectively.