Dedolomitization experiments were run at 300 bars for 7 to 8 weeks at 100°, 150° and 200°C in chloride brines containing 2 m (molal) Na, 0.5 m Ca, and (except for 1 experiment) 0.02 m Sr. The "ordered" dolomite dissolved and low-Mg calcite back precipitated as a replacement and/or rim cement of equant spar calcite crystals 10 to 50 μm in diameter. Between 100° and 200°C the mode of calcite formation changed from direct precipitation into void space produced by dissolving dolomite to pseudomorphic replacement of dolomite. The dedolomitization textures formed in these experiments are similar to those found in the rock record. Solution compositions were monitored as a function of experimental time and imply that calcite-dolomite equilibrium was approached at 100°C and reached at 150° and 200°C. The final aqueous molality ratios of Ca:Mg were 14.2, 23.5, and 35.4, respectively, at 100°, 150°, and 200°C. The respective activity ratios, computed using Pitzer's model, were 14.3, 23.6, and 37.0, representing estimated equilibrium constants for the reaction: Mg2+ + 2 CaCO3 = CaMg( CO3) 2 + Ca2+. The estimated standard state enthalpy of reaction is 3.6 kcal between 150° and 200°C. The empirical Sr distribution coefficient, ( Sr/Ca) solid( Sr/Ca) liquid, for the precipitated calcite was determined from micro-probe analyses of the calcite and solution data. The average coefficient values increased with increasing temperature, being 0.027 (±0.008 = s. d. of 19 measurements), 0.048 (±0.017 = s. d. of 19 measurements), and 0.063 (±0.013 = s. d. of 32 measurements), respectively, at 100°, 150°, and 200°C.