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Diffusional oxygen exchange between quartz and water was studied by determining 18O profiles in the quartz using an ion microprobe. Inverse error function plots of the data are linear as required for a diffusion mechanism. One natural and two synthetic samples were used, and although they had different impurity concentrations (OH−, Na, and Al per 106 Si atoms are 7–4000, 35–835, and 67–127, respectively), the D values were the same within experimental uncertainty. Diffusion parallel to c is about 2 orders of magnitude faster than that normal to c at 700°C, and diffusion normal to the rhombohedron is intermediate. The data indicate a change in slope on an Arrhenius plot at the α‐β boundary. At 100‐MPa (1 kbar) water pressure the preexponential factors (Do, in square centimeters per second) and activation energies (Q, in Kilojoules per mole) for α quartz (500°–550°C) are as follows: Do = 190 and Q = 284 parallel to c, and Do = 8 × 10−2 and Q = 238 normal to the rhombohedron. For β quartz (600°–800°C), Do = 4 × 10−7 and Q = 142 parallel to c, Do = 9 × 10−7 and Q = 155 normal to the rhombohedron; and Do = 1 × 10−4 and Q = 234 normal to c. D increases with PH 2 O, and between 25 and 350 MPa log D versus logƒH 2 O has a slope of approximately 1.1. These D values are all several orders of magnitude larger than previously reported values for oxygen diffusion in water‐free systems.