The mechanisms of frictional sliding in faulted Westerly granite were studied in two ways. Firstly, the experimental activation energy was measured from 300 to 700 °C at 2.5 kbar (2.5 × 105 kPa) pressure and sliding rates from 10−5 to 10−2 cm/s. Secondly, fault samples were examined with an optical and a transmission electron microscope. Below 500 °C the activation energy was about 30 kcal/mol (1.3 × 105 J/mol). The fault gouge was porous and consisted of angular randomly oriented grains. The quartz and the feldspars were unstrained, similar to the grains in a room-temperature fault. Above 500 °C the activation energy increased to about 85 kcal/mol (3.6 × 105 J/mol). Plasticity in quartz about 500 °C was observed optically by the presence of highly strained gouge grains and with the transmission electron microscope by a marked increase in dislocation density from 3 × 108 cm−2 initially to greater than 1011 cm−2 at 700 °C. The quartz grains away from the fault were strain-hardened with inhomogeneously distributed, dense tangles of dislocations. In contrast, the small grains (<10 μm) in the gouge contained a low density of dislocations. The feldspars showed no sign of plasticity up to 700 °C. Biotite and muscovite were plastic at all temperatures, forming thin ribbons along slip surfaces in the fault zone. Glass was not identified in any of the faulted samples studied.