Grinding temperatures are measured using an infrared sensor in ferrite and steel. For reference purposes, temperatures are also measured in a reduced model for grinding which consists of a single diamond grain sliding across the workpiece surface at high speed. The results include temperature as a function of sliding velocity, rate of temperature decay as the grain moves away from contact, and histograms of the frequency that grains on the grinding wheel attain a given temperature. It is found that temperature measurements can be used to detect out-of-roundness in the wheel. Finally, a simplified two-dimensional model based on a heat flux moving with constant velocity gives reasonably good agreement with experiment.