The influence of calcium ions on the adsorption of 1-hydroxy-ethane-1,1-diphosphonic acid (HEDP) on Fe electrodes has been examined. Surface analytical methods (XPS and XAES) have been applied in order to obtain direct information on the structure of the surface layer. On the surfaces of electropolished samples, various Fe2+, Fe3+ and FeOOH compounds as well as the metallic state of Fe could be identified in the surface layers. HEDP was adsorbed on the iron surface in almost the same extent, when the basic solutions were bidistilled water or 0.5 mol dm-3 NaClO4, respectively. The presence of Ca2+ cations increased the quantity of phosphonate adsorbed on the metal surface considerably. Varying the Ca/HEDP molar ratio in the solution between 0.5 and 4.0, the Ca/HEDP molar ratio on the metal surface remained under a limit close to one, corresponding to what could be expected on the basis of the results obtained by radiotracer method.