AbstractForward recoil spectrometry is shown to be a useful technique for measuring diffusion of d‐polymer chains in h‐polymer melts. Concentration profiles of a deuterated diffusing species may be determined with a depth resolution of 80 nm and a sensitivity of 0.1 vol % d‐polymer in h‐polymer. Consequently diffusion coefficients as small as 10−16cm2/s can be readily measured. If polymer chains diffuse by a reptation mechanism, the concentration profile ø(x) of diffusing polydisperse polymer should be quite different from øm(x), the Fickian solution, which one obtains for monodisperse polymer. This idea was tested by measuring ø(x) of polydisperse d‐polystyrene (d‐PS) diffusing into h‐PS. The results are in excellent agreement with the ø(x) predicted from the reptation model and the experimentally determined molecular