The fractionation of oxygen isotopes between natural analcime (~100 μm) and water has been determined at 300, 350, and 400°C at fluid pressures ranging from 1.5 to 5.0 kbar. Isotope ratios were obtained for the analcime framework, the channel water, and bulk water. Analcimes from Surtsey (145°C), DSDP Hole 417A (30 to 55°C), and Guam (25°C) were used to constrain the fractionation factors below 300°C. Analcime channel water exchanged completely with external water in all runs. Although some retrograde exchange may have occurred during quenching, the results indicate that the channel water is depleted in 18O relative to bulk water by a constant value of ~5%, nearly independent of temperature. Analcime is the first hydrated mineral found to have water of hydration depleted in 18O. Analcime framework oxygen exchanged 80, 90, and 96% at 300°C for 412 h, 350°C for 178 h, and 400°C for 120 h, respectively. Equilibrium Δ18O (%) are as follows: 2.9 (400°C), 4.5 (350°C), and 5.8 (300°C) for the experimental runs and 12.2 (145°C) and 24.2 to 28.2 (30-55°C) for the empirical data. The analcime-water fractionation curve is within experimental error of that of calcite-water. The exchange had little effect on grain morphology and does not involve recrystallization. This is the fastest exchange observed for a silicate. The rapid exchange rates indicate that zeolites in active high-temperature geothermal areas are in oxygen isotopic equilibrium with ambient fluids. Once calibrated, zeolites may be among the best low-temperature oxygen isotope geothermometers.