The solubility of portlandite was measured in H2O and aqueous acetate solutions of varying concentration (1-10 mmolal) at temperatures from 100-350°C and 500 bars pressure. Dissolved Ca concentrations increased with decreasing temperature and increasing dissolved acetate concentration. Using known thermodynamic data for portlandite, H+, OH-, Ca++, CH3COO-, CH3COOH0, and H2O(1), stability constants for CaOH+ and CaCH3COO+ complexes were determined. Log K values for the reaction Ca++ + H2O(1) = CaOH+ + H+ are, respectively, -10.04, -8.20, -6.88, and -6.35 at 100, 200, 300, and 350°C and 500 bars pressure and for the reaction CaCH3COO+ = Ca++ + CH3COO- are, respectively, -2.53, -3.72, -4.59 at 200, 300, and 350°C and 500 bars pressure. These results indicate that the stabilities of CaOH+ and CaCH3COO+ complex increase with increasing temperature. In the acetate-free experiments, CaOH+ is the dominant form of dissolved Ca in equilibrium with portlandite at 100-350°C and 500 bars, while in the acetate-rich experiments (10 mmolal acetate), Ca++ and CaOH+ are the dominant forms of Ca in equilibrium with portlandite at low temperature (100-200°C) and CaCH3COO+ and CaOH+ are the dominant forms at relatively high temperature (200-350°C). Metal-acetate complexing has long been suggested as an important mechanism for mobilizing base metals during the formation of ore deposits in organic-rich environments. Due to the stability of CaCH3COO+ complex in Ca-bearing fluids at elevated temperatures and pressures, however, the effectiveness of dissolved acetate to enhance base metal sulfide solubility is limited.