The transient heating by radiation of a stagnant layer of water is studied both analytically and experimentally. Solar radiation (insolation) was simulated by tungsten filament lamps in parabolic reflectors of known spectral characteristics. The volumetric rate of internal absorption of radiation was predicted by considering spectral absorption, scattering, and multiple forward scattering by the water according to the model of Viskanta and Toor (1972). The transient temperature distribution was predicted by solving the one-dimensional energy equation analytically in closed form after first linearizing the boundary condition at the air-water interface. The analytical model was verified on the basis of laboratory experimental data obtained from transient temperature measurements made in pure water contained in a glass wall test cell using a Mach-Zehnder interferometer. Comparison of experimental data with the predicted temperature distributions showed good agreement, thus verifying the radiation and total energy models. It was determined that the surface boundary condition and internal radiant heating rate of water must be correctly specified in order to model thermal stratification of stagnant water by radiation properly.