Oxygen isotope data for the Grey River tungsten prospect, Newfoundland, Canada, indicate a progressive depletion in δ18Ofluid during mineralization. Early veins with pegmatitic affinities were deposited at 470 °C and pressures greater than 1 kbar (100 MPa), from a fluid with a δ18O composition of 7.4‰, presumed to be of magmatic origin. Successive vein deposition, at progressively lower temperatures and pressures, culminated in the precipitation of wolframite-bearing veins at a temperature of 300 °C and pressures of 150–320 bar (15–32 MPa), from a low salinity fluid with a δ18O composition in the range 3.2–1.6‰.Low values of δ18O (and δDfluid) are recorded in many vein tungsten deposits and are normally interpreted as reflecting mixing of isotopically light meteoric fluids or formation brines with magmatic fluids. However, fluid-inclusion evidence for the Grey River mineralization indicates that a 40 mol% CO2 loss occurred by immiscibility and retrograde boiling of the hydrothermal fluid between 420 and 300 °C. Such a chemical change would have significantly altered the oxygen isotopic character of the hydrothermal fluid since CO2 fractionates 18O relative to coexisting water by ~10‰ at 400 °C and ~14‰ at 300 °C. Calculations using available CO2–H2O fractionation factors reveal that up to a 7‰ depletion in δ18O of the residual aqueous fluids may occur as a result of the 40 mol% CO2 loss from the hydrothermal fluid.