Hydrothermal treatment experiments have been performed on single crystals of San Carlos olivine at 1100¿ to 1300¿C and 300- and 1500-MPa confining pressure, with the oxygen fugacity around the specimen constrained neat the Fe/FeO buffer. The diffusion coefficients for the hydroxyl species giving rise to the sharp band and broadband features in the infrared spectrum of hydrothermally treated olivine have been determined to be greater than 10-1-0 m2 s-1 under all pressure and temperature conditions tested; the solubility of the hydroxyl species seems to vary between crystals but to show little variability for specimens from the same crystal over the temperature range investigated at 300-MPa confining pressure. Crystals hydrothermally treated at 1300¿C, 300-MPa confining pressure, and 10-5 s-1 strain rate as a factor of 1.5 to 2.5 weaker than those treated in an anhydrous environment. Microstructural investigation suggest that this reduction in strength is due to enhancement of the rate of climb of dislocations in the ''wet'' experiments. First-order calculations suggest a stress exponent of n~2.5 and a dislocations in the ''wet'' experiments. First-order calculations suggest a stress exponent of n~2.5 and a water fugacity exponent of m~1/5 for deformation under hydrous conditions, assuming that the deformation obeys a power law relation.