The uniformity of the product of thickness and density (i.e., mass per unit area) of materials of constant mass absorption coefficient is determined by measurements of the variation in gamma‐ray transmission. The radiation is detected with a scintillation detector and vibrating‐reed electrometer. For a given incident intensity, maximum sensitivity is attained when a source is chosen which emits gamma rays whose mean free path in the material under investigation equals the thickness, assuming the detector response is a slowly varying function of quantum energy. This optimum condition for cobalt‐60 radiation obtains for thicknesses of 3, 1, and ½ in. of Al, Fe, and U, respectively; using a one‐curie source, variations of 0.01 percent in the uniformity of plates of these thicknesses have been measured. The method may be applied also to the scanning of curved surfaces and extended materials where the source and detector cannot permanently be fixed with respect to each other. For this latter case, the problem of compensating for the effect of misalignment of source and detector is considered.