Strontium chloride added to aqueous suspensions of metastable calcium citrate tetrahydrate increased calcium ion activity measured electrochemically without transition of metastable tetrahydrate to stable calcium citrate hexahydrate as shown by DSC. Calcium activity increase was explained by lower solubility of strontium citrate pentahydrate formed (8.9 × 10−4 M at 25 °C) increasing with temperature compared to calcium citrate tetrahydrate (1.6 × 10−3 M) decreasing with temperature. Strontium binding to citrate was found endothermic, ΔH0 = 45 kJ∙mol−1 at 25 °C, while calcium binding shows variation from ΔH0 = 94 kJ∙mol−1 at 10 °C becoming exothermic above physiological temperature with ΔH0 = −9 kJ∙mol−1 at 45 °C as determined from temperature and concentration variation in electric conductivity. These differences in solution thermodynamics and pH effect on complex formation between calcium and strontium citrate are discussed in relation to biomineralization.