Rheological properties of olivine and its high pressure polymorphs, both dry and hydrous, have been studied by monitoring x-ray diffraction peak broadening as a function of pressure, temperature and time. The measurements were carried out up to a pressure of 10 GPa and a temperature of 600 ¿C for olivine and wadsleyite, and a pressure of 20 GPa and a temperature of 1000 ¿C for ringwoodite. The observations indicate that olivine is much weaker than the other two minerals for this range of conditions, and furthermore, water weakens olivine dramatically but only slightly weakens wadsleyite and ringwoodite. When the temperature increases from 25 ¿C to 400 ¿C the yield strength in the olivine drops by 39% without water and 62% with water, however there is hardly any change in strength for wet or dry wadsleyite and ringwoodite. ¿ 1998 American Geophysical Union