A calorimeter was made to determine the relative amount of gamma heating in watts per gram in different materials as a function of thickness and atomic number. The experiment was performed in the NASA Plum Brook Mockup Reactor, which has a typical light-water test reactor gamma-source spectrum. Carbon, aluminum, zirconium, tin, and lead specimens in slab geometry were irradiated. The results showed no significant difference in the gamma heating in carbon and aluminum, but the heating in the other materials was greater than that in aluminum and carbon. The smaller thicknesses had the greater heating. The calorimeter was also used to determine the gamma-heating effect in an irradiation experiment mockup having cylindrical geometry. The result showed good agreement with an expected value obtained from the slab geometry data. A theoretical analysis of the relative gamma heating was made using a one-dimensional multigroup transport program. It was concluded that the analysis and measurements agreed qualitatively and that quantitative differences were attributable mostly to geometrical effects. The results of this study are believed to be applicable to both nuclear reactor experiment designs and other reactor problems.