The Doppler effect in 238U capture and 235U fission has been measured by means of a foil activation technique in the fast-neutron spectrum core of the Mixed Spectrum Critical Assembly. Experimental results were obtained for two 238U foil thicknesses and one 235U foil thickness. The amount of scattering material between the foil and surrounding core fuel was varied to determine the effect on the Doppler measurement of change in the incident flux fine-energy structure in the resonances. In this experiment, only the foil is heated, while the core fuel remains at room temperature. The experiment is analyzed by means of the collision-probability method which is used to develop an expression for the resonance integral of a thin absorber which is separated from a homogeneous reactor fuel region by a purely scattering medium. The general expression for the foil resonance integral is simplified and numerical results are presented for the case in which the dominant resonances are weak; that is, for a fast reactor in which the 0.5 to 3.0-keV energy region dominates the 238U Doppler effect. The measured 238U Doppler effect expressed as the ratio typically was of the order of 0.015 ±0.002. This was a factor of 2 higher than that calculated using a neutron energy spectrum derived from “nominal” material cross sections. Presently available cross sections in the energy range of interest are sufficiently uncertain so that it is possible to infer from them “hard” or “soft” neutron energy spectra such that the value oi R-l varies by a factor of 2. The measured values for 238U agreed quantitatively with those found from the “soft” neutron energy spectrum. Within the precision of the measurement no 235U Doppler effect was observed. The calculated 235U Doppler effect was smaller than the sensitivity of the experiment, thus, within its precision (± 0.002), the measurement confirms the theory.