The impact assessment of two phosphonate ligands: HEDP [1-hydroxyethane-1,1′-diphosphonic acid] and ATMP [aminotris(methylenephosphonic acid)], on the speciation of various divalent cations (Cu, Zn, Cd and Ni), was undertaken taking into account both water alkalinity and hardness. In order to model the phosphonate/carbonate/calcium/metallic cation systems, a critical study of the literature concerning stability constants of soluble and insoluble species was carried out. The model systems show that speciation of metallic phosphonate complexes depends on the alkalinity and with more particularly, the hardness of water. Copper and zinc speciation in mineralized waters is only modified by ATMP, the strongest complexing ligand. HEDP mainly complexes calcium in such systems and does not interfere with heavy metal speciation.