Laboratory experiments on simulated quartz gouges at a temperature of 927 ¿C, a confining pressure of 300 MPa, and a pore water pressure of 200 MPa demonstrate the interactions between cataclastic and dissolution--precipitation processes and their effects on the strength and mechanical behavior. Significant porosity reduction and microstructures, including grain interlocking, grain interpenetration, and widespread growth of euhedral-shaped grains, indicate the operation of dissolution--precipitation processes. Under conditions which favor the activity of dissolution--precipitation processes, i.e., for small grain size or at slow displacement rates, deformation is distributed across the whole gouge layer, and strength increases continuously with increasing displacement, so that coefficients of friction reach to 0.7. In contrast, for specimens with large grain size sheared at fast displacement rates, deformation is localized along a gouge-forcing block interface after a small amount of displacement and results in slip softening and subsequent quasi-stable sliding with coefficients of friction as low as 0.45. Analysis of our results indicates that where the kinetics of dissolution--precipitation is sufficiently fast to partly accommodate shear strain, the mechanical behavior and strength of quartz gouge can be characterized by slip hardening and high strength with velocity strengthening; in contrast, where cataclastic processes predominantly accommodate shear strain, the mechanical behavior and strength can be characterized by slip softening and subsequent localized sliding with low strength and velocity strengthening. ¿ 2000 American Geophysical Union