Synthetic quartz aggregates of low porosity have been fabricated from natural quartz powder, impure silica gel, and high-purity silicic acid by hydrothermal isostatic pressing at 300 MPa. The resulting specimens have water contents comparable to those of natural quartzites and have been used for deformation tests. The flow stress at 1200-1300 K was found to vary in an inverse relationship to the water pressure estimated under the assumption that the measured water content homogeneously filled the measured porosity. It is questionable that the natural-quartz-origin specimens were fully equilibrated in respect to the activity of the water, but equilibration has probably been achieved in the amorphous-silica-origin specimens, which crystallized under the experimental conditions. In the deformation of the latter materials, the experimental activation energy at water pressures approaching the confining pressure was found to be about 150 kJ mol-1 for both. However, the impure gel-origin material gave a stress exponent of about 2.3, whereas the high-purity silicic-acid-origin material gave a stress exponent of about 4, in spite of the grain sizes being about 90 μm and 20 μm, respectively. It is concluded that while the impure specimens have higher intragranular strengths, there is also a significant contribution of grain boundary processes to the strain in them which is absent in the high-purity specimens. ¿ American Geophysical Union 1992