Neutron rethermalization has been studied, experimentally and theoretically, in a system intended to reproduce the conditions of the Kottwitz problem. Terphenyl and light water were used as the moderators at the temperatures 223 and 11°C, respectively. The energy spectrum of the angle-dependent neutron flux perpendicular to the plane discontinuity was measured in terphenyl as a function of the distance from the discontinuity by means of an extraction channel and a choppertime-of-flight analyzer. The spatial behavior of the flux-weighted average energy was determined by fitting a Maxwellian to each measured spectrum using the method of the least squares. This spatial behavior could be satisfactorily described by a simple one-exponential function for distances exceeding 3 mm (measured from the discontinuity). The least-squares fit gave a value of 11.3 mm for the relaxation length. The theoretical calculations were performed by an approximate method using the “two overlapping-groups” approximation for the energy dependence. For the angular dependence of the flux, both the Pn(n =1,3)- and the DPn(n = 1)-approximation was used. Three different scattering models were used for each moderator. Rather good agreement with the theory and the experiment was achieved as to both the form of the spectrum and the spatial behavior of the average energy.