Enthalpies of mixing [Δ H 298 mix] of aragonite and strontianite to form Ca x Sr 1− x CO 3(s) solid solutions were obtained at 298 K from drop-solution enthalpies [Δ H 975 ds] of pellets of the respective solid solutions into molten 2PbO · B 2O 3 at 975 K. The measured Δ H 298 mix values are positive for all measured values of x, are nearly symmetric around x = 0.50, and reach a maximum value of +3.82 ± 0.94 kJ mol −1. Previous electrochemical studies have reported that Δ G 298 ex values are also positive over the range 0.0 < x < 0.9 and reach a maximum value of +3.0(±1.6) kJ mol −1 at x ≈ 0.7. The general similarity between the Δ H 298 mix and the Δ G 298 ex values indicates that the excess entropy of mixing is small or zero, consistent with the regular-solution treatment. Within this regular-solution treatment, the interaction parameter is W = 13.5(±1.3) kJ mol −1, which yields a very narrow range of stable miscible compositions at Earth surface conditions. Compositions of aragonite or strontianite with even a few percent impurity are not stable and will unmix to form a mechanical mixture of Ca-rich strontianite and a Sr-rich aragonite. It is, in general, difficult to accurately estimate the compositions of coexisting, miscible solids such as the Ca x Sr 1− x CO 3 solids at Earth-surface conditions. The predictions are quite sensitive to small uncertainties in the data and such estimates are probably beyond the capabilities of either electrochemical cell measurements or calorimetry.