An ion-beam technique based on the nuclear reaction d(3He,p) 4He has been applied to measurements of polymer interdiffusion. A diffused bilayer of deuterated polystyrene and unmodified polystyrene is analysed by a beam of 3He ions and a concentration-profile of deuterium is extracted by analysis of the energy-spectrum of the emitted energetic protons. This technique can be used to measure diffusion over several microns with a depth-resolution of 20 nm. Reptation has been confirmed as the mechanism governing polymer self-diffusion over the molecular weight range 90,000 to 1,800,000. The 3He-d method has also been used to study mechanisms of diffusion in ionomers, a class of polymers containing pendant ionic groups. The effect of ionic concentration on diffusion shows that the critical concentration for the formation of diffusion-inhibiting ionic aggregates lies in the range 2.8 - 5 weight % of ionic material. Ionomers with concentrations below this range diffuse by reptation at an effective temperature equal to the difference between the annealing temperature and the glass-transition temperature.