We describe a method based on nuclear reaction analysis, using the reaction2H(3He,4He)1H, (Q=18.352 MeV) to determine composition profiles of deuterated polymer chains in thin films. By detecting the emitted &agr; particles (4He) at forward angles (30°) we are able to achieve a spatial resolution of 7 nm half width at half maximum (HWHM) at the deuterated sample surface, and 15 nm HWHM at a depth of some 130 nm. We use our method to probe initial diffusional broadening at the interface between deuterated and protonated polystyrene films. Our measured profiles are in close agreement with earlier measurements (over larger spatial scales) and with mean field models for the diffusional process in this system.