The water-related chemical parameter that affects dislocation creep in quartzite has been determined from variations in sample strength and microstructure with chemical environment in buffered deformation and hydrostatic annealing experiments. Samples were weld-sealed in double capsules; fH2, fO2, fH2O, and aH+ were buffered using solid oxygen buffers, AgCl or CO2. Black Hills quartzite was deformed at 900 ¿C and 1.5¿10-5 s-1. Two samples were deformed at ~1700 MPa confining pressure at constant fH2O and aH+, with fH2 and fO2 varying over 8 and 15 orders of magnitude, respectively. Both samples deformed by climb-accommodated dislocation creep with flow stresses of 300 MPa. Two additional samples were deformed at ~700 MPa at constant fH2O lower than for the 1700-MPa samples, with aH+ varying over 2 orders of magnitude. Both samples faulted with a peak strength of ~800 MPa. These four experiments suggest no dependence of dislocation creep strength on fH2, fO2 or aH+; instead, a strong dependence of strength on fH2O is inferred. Previously deformed samples of Heavitree quartzite were hydrostatically annealed for 4 days at 800 ¿C and 1200 or 500 MPa confining pressure, varying aH+ and fH2O over 2.5 and 1 order of magnitude, respectively. The microstructures of these samples show increased rates of dislocation climb and grain boundary migration with increasing fH2O but no dependence on aH+. These buffered experiments indicate that dislocation creep is affected by fH2O alone and suggest that the exponent for the fH2O term in the power law creep flow law is >2. ¿ American Geophysical Union 1996