The influence of water on diffusion creep of olivine aggregates was investigated by performing high-temperature creep experiments under both hydrous and anhydrous conditions. Deformation experiments were conducted on fine-grained samples using a gas-medium apparatus at confining pressures of 100 to 450 MPa and temperatures between 1473 and 1573 K. Water was supplied by the dehydration of talc, which occurs near 1075 K. Water fugacities of ~85 to 520 MPa were obtained by varying the confining pressure under water-saturated conditions. Under both hydrous and anhydrous conditions deformation was dominated by grain boundary diffusion. At a water fugacity of ~300 MPa, samples crept ~5 times faster than those deformed under anhydrous conditions at similar differential stresses and temperatures. Within the range of water fugacity investigated, the strain rate is proportional to water fugacity to the 0.7 to 1.0 power, assuming values for the activation volume of 0 to 20¿10-6 m3/mol, respectively. We propose the following point defect model to explain this water-weakening effect: In going from an anhydrous to a hydrous environment the charge neutrality condition changes from <FeMe*>=2<VMe″> to <FeMe*>=<HMe'>. As a consequence, for olivine aggregates the concentration of silicon interstitials, the rate of silicon diffusion, and therefore the rate of diffusion creep increase systematically with increasing water fugacity (i.e., OH concentration). ¿ 2000 American Geophysical Union