In this paper, the properties of precipitated “mixed” calcium and magnesium phosphonate scale-inhibitor (SI) complexes formed by nine common phosphonate species are investigated. These complexes are of the form SI–CaN1–MgN2, where the stoichiometry (Ca2+/SI and Mg2+/SI molar ratios; i.e., N1 and N2, respectively) in various precipitates is established experimentally, and the effect of solution pH on the stoichiometry is determined. Static precipitation tests were performed by varying the amounts of Ca2+ and Mg2+ present in the system (at a constant ionic strength) at test temperatures ranging from 20 to 95°C and at a fixed [SI]=2,000 ppm. The stoichiometries of the solid precipitates were determined by assaying for Ca2+, Mg2+, and P in the supernatant liquid, under each test condition, by inductively coupled plasma spectroscopy. It is shown experimentally that for all nine phosphonates tested, these stoichiometries (i.e., N1 and N2 in SI–CaN1–MgN2) depend on (1) the nature of the SI (i.e., Mg2+ binding sites per molecule); (2) the solution pH, which affects the speciation of the SI; (3) the relative magnitude of the SI binding constants to Ca2+ and Mg2+ at the test pH (Kb1 and Kb2, respectively); and (4) 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-MS (Shaw et al. 2012c) and SPE-164051-PA (Shaw and Sorbie 2014a)]. These functionality data are useful in a practical sense when performing SI-squeeze treatments in combination with Ca2+. In addition, the precipitation behavior of the various compounds is modeled theoretically by developing and solving a set of simplified equilibrium equations. Very good agreement is seen between the modeling and experimental results. Such models can be used directly in the simulation of field phosphonate precipitationsqueeze treatments to design and optimize squeeze lifetimes.