The influence of calcium and zinc ions on the corrosion inhibition effect of 1-hydroxyethane-1,1-diphosphonic acid (HEDP) on carbon steel has been investigated by electrochemical techniques, X-ray photoelectron spectroscopy, and atomic force microscopy. Addition of calcium or zinc ions greatly increased the inhibition efficiency of HEDP in a synergistic manner. The highest inhibition effect was obtained for molar ratios of Ca/HEDP = 0.5 and Zn/HEDP = 3 for 3 x 10(-4) mol dm(-3) HEDP. The corrosion inhibition mechanism in the presence of these additives proved to be different from that with HEDP alone and was related to the formation of different complex species between HEDP and cation additives. The frequency response of the studied system was described by a transfer function containing two relaxation time constants corresponding to the charge-transfer process and the porous oxide layer. Higher zinc and phosphorous content in the inhibitor layer and a continuous change in the O-Fe and HO-Fe ratio were achieved by increasing zinc concentration in the solution (in the range Zn/HEDP = 0.5-3) as supported by XPS measurements. Changes in the chemical structure were established in the oxids-hydroxide layer developed spontaneously on the steel surface. The synergistic effect of inhibitor compounds appeared also in surface morphology, leading to the development of smooth, i.e., dense, protective layers as revealed by atomic force microscopy.