In oilfield applications, various divalent cations are present in field produced waters, e.g. Ca2+, Mg2+, Sr2+ and Ba2+. The interaction of these cations with phosphonate scale inhibitors (SI) is important both for scale inhibition and inhibitor retention mechanisms. In this paper, the properties of precipitated "mixed" Ca2+/M2+phosphonate SI complexes formed by 9 common phosphonate species are investigated, where M2+ may be Mg2+, Sr2+, or Ba2+. These 2 and 3 metal ion complexes with SI are of the form SI__ CaN1_SrN2 and CaN1_MgN2_BaN3_, where the stoichiometry N1, N2 and N3, are the respective (M2+/SI) molar ratios. Static precipitation tests were performed varying the amounts of Ca2+ and M2+ present in the system (at a constant ionic strength), at 950C, pH 8.5, and fixed [SI] = 2,000ppm. The stoichiometries of the solid precipitates were determined by re-dissolving the precipitated deposits in acidified distilled water (DW/HCl), and then assaying for Ca2+, M2+, and P directly, under each test condition, by Inductively Coupled Plasma (ICP) spectroscopy. It is shown experimentally that, for all 9 phosphonates tested, these stoichiometries depend on the number of M2+ binding sites per molecule, solution pH, the relative SI binding constants to Ca2+ and M2+ at the test pH, and the solution molar ratio of M2+/Ca2+.