In studies of epithermal precious and base metal ore deposits, estimates of salinity (total dissolved salts) are frequently in error when based on fluid inclusion ice melting measurements in the absence of an independent determination of the CO2 content of the inclusion fluid. For a fluid of known composition, the melting point of ice (Tin) may be calculated from Tm= --• Kimi where Ki is the molal freezing (or melting) point depression constant and mi is the molality of a component i (i = Na +, K +, CI-, CO2, etc.). For fully dissociated solute species such as CI-, in the salinity range considered here, K = 1.72 Kelvin/molal, and for undissociated, nonpolar species such as CO•, K --1.86 Kelvin/molal. Fluid inclusion ice-melting data from New Zealand geothermal fields correlate well with values calculated using the above equation and the measured compositions of discharges from wells from which the inclusion samples were obtained. Loss of the dominant dissolved gas, CO•, during boiling at depth results in large, systematic decreases in apparent salinity (in terms of Tm) in the Broadlands field. Misinterpretation of fluid inclusion freezing data may lead to substantial errors in the reconstruction of the physico-chemical environment of ore formation in fossil systems. For example, in the absence of CO• analyses, inclusion fluids similar in gas content to the Broadlands geothermal fluid (NaC1 = 0.2 wt %, CO2 up to 4.4 wt %) may be interpreted to have salinities of 0.85 wt percent NaC1, leading to errors of 23 percent in the estimated depth of formation at 280øC, -0.5 unit in the estimated pH of the ore fluid, and on the order of 200 times in the estimated solubility of an ore component such as lead. Such errors may be transmitted into subsequent estimation of fluid flux or duration of ore formation.A review of published fluid inclusion studies shows that true salinities of inclusion fluids from epithermal precious metal deposits are coincident with those from the majority of explored geothermal systems but that the salinities of inclusion fluids from epithermal base metal deposits are substantially higher, indicating the presence of an evolved, high-salinity fluid during ore metal transport. In the case of low-salinity epithermal deposits, the extreme range in gold to silver ratios between deposits can be accounted for by variations in the H•S to chloride ratio in the original hydrothermal fluid. These variations (primarily in the total gas) occur between active geothermal systems and can be recognized in epithermal deposits through fluid inclusion freezing and crushing studies.