Abstract Formation of the complex between [Pd(en)Cl2] (en = ethylenediamine) and 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP, H4L) was studied by pH-potentiometry, UV–Vis spectrophotometry and 31P NMR spectroscopy in aqueous solutions containing CKCl = 0.15 mol L−1 and 0.004 mol L−1, which corresponds to the concentration of chloride ions in intercellular and intracellular fluids. The formation