Effects of pressure on high-temperature, dislocation creep in olivine 2 SiO 4 ) aggregates have been determined under both water-poor and water-saturated conditions. New experimental data were obtained at pressures of 1-2 GPa under 'dry' and 'wet' conditions using a newly developed high-resolution dislocation density measurement technique to estimate the creep strength. These data are compared with previous data at lower and higher pressures to determine the pressure dependence of high-temperature dislocation creep in olivine aggregates. We find that the creep (...) strength † under 'dry' conditions increases monotonically with increasing pressure, whereas the creep strength under 'wet' conditions changes with pressure in a non-monotonic fashion: it first decreases rapidly with increasing pressure and then becomes less sensitive to pressure at above 1 GPa. Such behaviour can be described by the following formula: where the subscripts d and w refer to parameters for 'dry' and 'wet' conditions respectively. The present study gives A d = 10 6.1 - 0.2 s m 1 m n , , and r = 0 for 'dry' conditions, and A w m 10 2.9 - 0.1 s m 1 m n m r , , and r = 1.20 - 0.05 for 'wet' conditions . The large activation volume for 'wet' conditions can be interpreted as due to the additional contribution from the activation volume for dissolution of OH in the olivine structure . The value of r is consistent with a model in which creep in olivine is rate controlled by the motion of positively charged jogs through the diffusion of silicon via an interstitial mechanism. (shrink)