Viscous heating during magmatic flow may play a major role in eruption dynamics. In order to document viscous heating during deformation of magma, we have conducted a series of rheological experiments where viscous heating is directly monitored via thermocouples in high-viscosity magmas. We observe experimentally the strain rate dependence of viscous heating. Viscous heating becomes rheologically significant in the highly viscous lavas investigated at strain rates above ca. 10− 3 s− 1. A simple analysis shows that the temperature increase generated through viscous heating during deformation of melts with viscosities ranging from 108 to 1012 Pa·s can account for their apparent non-Newtonian rheology in these experiments. This thermal correction transforms apparent non-Newtonian, strain rate dependent rheology of magma to a Newtonian behavior over the range of conditions accessed in this work. In this manner, this study provides an experimental basis for separating the relative roles of structural relaxation and viscous heating in the generation of apparent non-Newtonian rheology at high strain rates. Here, viscous heating dominates and the observation of the structural onset of non-Newtonian behavior is precluded by a viscous heating-induced lowering of the Newtonian viscosity. The interplay of viscous heating and structural relaxation in melts in nature is discussed briefly.