Summary: A study of phase equilibria among the orthosilicates on the Ca₂SiO₄—CaMgSiO₄ join was undertaken with the use of hydrothermal techniques in the temperature range 600–900°C. at water pressures from 1000 to 15000 psi. Monticellite, merwinite, and Ca₂SiO₄ crystallize as pure phases throughout this temperature range, and intermediate compositions contain mixtures of the end members. Powder X-ray diffraction data obtained on the mixtures indicate no change in lattice dimensions from the end members. Hence, very little or no solid solution is indicated at these temperatures. Ca₂SiO₄ was crystallized as the γ-form below 675°C. (at 2000 psi) and the β-form above this temperature (presumably having inverted from the α′-form on cooling to room temperature). Infra-red absorption spectra obtained on the compounds γ-Ca₂SiO₄, β-Ca₂SiO₄, merwinite, and monticellite show characteristic differences in the pattern within the 9–13 micron region. All the data thus appear to confirm merwinite as a unique compound stable in the temperature range studied.