The velocities of compressional waves have been determined for several igneous and metamorphic rocks to temperatures of 500¿C at elevated confining pressures. At 2 kbar and between 25¿ and 300¿C, temperature coefficients (∂Vp/∂T)p for many of the rocks range between -0.5¿10-3 and -0.6¿10-3 km s-1¿C-1. At higher temperatures and 2 kbar the temperature coefficients show extreme variability, which is related to the opening of grain boundary cracks caused by anisotropic thermal expansion of the mineral components. Critical thermal gradients (dT/ dZ)c for a low-velocity layer in the continental crust at pressures of 5--8 kbar are between 10¿ and 14¿C/km. These values are probably high because of porosity; however, they are still lower than estimated crustal temperature gradients in normal and high heat flow provinces. Thus it is concluded that crustal velocity inversions produced by high temperature are likely to be common within the crust. It is significant, however, that the velocity decreases reported for some crustal low-velocity layers are much greater than the decrease which can be accounted for by temperature alone. At higher pressures (10--30 kbar), (dT/dZ)c=6.3¿C/km for dunite and eclogite, in excellent agreement with estimates from single-crystal data. The velocities in dunite as a function of temperature further support the conclusion that the observed increase in upper oceanic mantle velocity with age is a consequence of decreasing temperature.