Pyroxene geotherms under continents (F. R. Boyd, 1973, 1975) and ocean basins (I. D. MacGregor, 1974) are characterized by inflections to steeper thermal gradients at temperatures above 1050¿C. The inflections are interpreted as marking the lid of the low-velocity zone, and the steeper gradients are attributed to shear heating. Mechanical considerations suggest, however, that shear heating is likely to be negligible for viscosities and strain rates expected at the physical conditions of the inflections, and this suggestion seems to be borne out by reequilibration paths deduced for partially recrystallized specimens. With appropriate corrections applied to new and existing data for solid solution effects on temperature and pressure determinations the inflections disappear and are interpreted here as artifacts reflecting varitions in the degree of depletion with depth. The oceanic and continental pyroxene geotherms derived are in accord with those estimated on the basis of other geophysical and geochemical observations.