Oxygen isotope fractionations in the quartz-cassiterite-water and the wolframite-water systems were experimentally investigated in hydrothermal autoclaves at pressures of 300 to 800 bars. The isotopic exchange experiments involving cassiterite were carried out by direct exchange between mineral and water and/or by synthesizing cassiterite from SnCl 4 . 5H 2 O in given salinities of solution at temperatures of 250 degrees to 500 degrees C. The isotopic equilibrium fractionations between wolframite and water were approached by synthesizing wolframite, ferberite, and huebnerite from Na 2 WO 4 . 2H 2 O + FeCL 2 . 4H 2 O and/or MnCl 2 . 4H 2 O in a temperature range from 200 degrees to 420 degrees C. Significant temperature dependence is obtained from oxygen isotope fractionations in the quartz-cassiterite, quartz-wolframite, cassiterite-water, and wolframite-water systems. The results also indicate that wolframite and its end-member minerals have identical isotopic fractionation behavior at the same temperature. The present study further suggests that the effect of salinity on oxygen isotope fractionation in mineral water systems may not be significant at temperatures above 250 degrees C.