Holocene dolomite forming in supratidal carbonate sediments of Bonaire, Netherlands Antilles, is precipitated from hypersaline fluids in which the Mg:Ca ratio is elevated by evaporation and gypsum precipitation. Kinetic effects and water-vapor exchange between the humid atmosphere and hypersaline brines limit the δ18O values of the brines to between +0.7‰ and +2.1‰ standard mean ocean water. The mean δ18O value of dolomite is +1.0‰ Peedee belemnite (n = 8), which is lower than values of dolomite precipitated from hypersaline brines in less humid environments. Bonaire dolomite is also isotopically lighter than dolomites interpreted to have formed in normal marine waters and mixed marine and/or meteoric waters. Calculated fractionation between dolomite and calcite formed from Bonaire brines is between 1.5‰ and 3.5‰. The high end of this range is consistent with previously proposed mineralogic fractionations of 3‰ ±1‰ and 3.2‰ between dolomite and calcite.