In this paper, the properties of precipitated “mixed” calcium and magnesium phosphonate scale inhibitor (SI) complexes formed by 9 common phosphonate species are investigated. These complexes are of the form SI_ CaNl _MgN2 where the stoichiometry (Ca2+/SI and Mg2+/SI molar ratios, i.e. N1 and N2) in various precipitates are established experimentally, and the effect of solution pH on the stoichiometry is determined. Static precipitation tests were performed varying the amounts of Ca2+ and Mg2+ present in the system (at a constant ionic strength), at test temperatures ranging from 20°C to 95°C, at a fixed [SI] = 2, 000ppm. The stoichiometries of the solid precipitates were determined by assaying for Ca2+, Mg2+, and P in the supernatant liquid, under each test condition, by ICP spectroscopy. It is shown experimentally that, for all 9 phosphonates tested, these stoichiometries (i.e. N1 and N2 in SI_ CaN1 _MgN2) depend on (i) the nature of the SI (i.e. M2+ binding sites per molecule); (ii) solution pH, which affects the speciation of the SI; (iii) the relative magnitude of the SI binding constants to Ca2+ and Mg2+ at the test pH (Kb1 and Kb2, respectively); and (iv) the solution molar ratio of Mg2+/Ca2+. It is found that, as pH increases, the combined molar ratio of Ca2+ and Mg2+ to SI, i.e. Nt = N1 + N2 in the complex, increases up to a theoretical maximum, Nt,max, depending on the chemical structure of the phosphonate (corroborating earlier work, SPE 155114, SPE 164051). In addition, the precipitation behaviour of the various compounds is modelled theoretically by developing and solving a set of simplified equilibrium equations. Very good agreement is seen between the modelling and experimental results. Such models can be used directly in the simulation of field phosphonate precipitation squeeze treatments in order to design and optimize squeeze lifetimes.