Analytical scanning electron microscopy has been used to determine the major element compositions of some natural and artificial silicate glasses and their microcrystalline equivalents derived by the frictional melting of intermediate to acid protoliths. The data show that the matrices of the friction melts (which cool to form pseudotachylytes) are relatively basic and hydrous, even when their protoliths are intermediate to acid. This is because frictional fusion involves the selective comminution and nonequilibrium melting of minerals based on their individual mechanical properties and melting points, not the formation of minimum melts through equilibrium mineral interaction. This means that hydrous ferromagnesian minerals (e.g., micas and amphiboles) melt preferentially to form the liquid matrix, while feldspars and especially quartz more readily survive as clasts. Pseudotachylytes generated by frictional melting are therefore not bulk melts, and as clast-melt suspensions, they cannot be considered as simple Newtonian fluids. The calculated viscosities of the friction melts are low. For example, at 1200°C, most friction melts possess zero-shear suspension viscosities of 102–104 dPa s (1 dPa s = 1 P). This is equivalent to the viscosities of tholeiitic and alkaline basaltic magmas at the same temperature. These viscosities are maximum determinations because, as clast-melt suspensions, friction melts may undergo shear thinning and exhibit pseudoplasticity at high shear rates (i.e., during slip on a fault surface). Contrary to earlier suggestions, where the bulk melting of intermediate to acid protoliths was believed to result in the generation of viscous friction melts that could act to inhibit continued sliding, this work shows that most pseudotachylytes are partial melts possessing low viscosities. The formation of highly fluid suspensions during slip may have profound effects on the dissipation of stored strain energy in the rocks surrounding a fault. Interface lubrication could facilitate an increase in the slip rate and the rate of energy dissipation. This would be manifest as an increase in high-frequency seismic wave radiation and vibrational.