Abstract A new method is described which allows the determination of diffusion fluxes during ion implantation and the measurement of diffusion coefficients down to about 10 −11 cm 2/s. The permeation of deuterium through stainless steel has been investigated for the case of 8 keV D 2 + implanted into foils of 25 μm thickness at temperatures between 70°C and 95°C. The permeating deuterium was collected in a Ti layer of 2000 Å thickness on the rear side of the foil and detected by means of the D( 3He, p) 4He nuclear reaction. For high analysing fluences, the results are influenced by the damage introduced by the 3He beam. The diffusion coefficients derived from the time dependence of the arriving D fluence are in agreement with the extrapolation of literature data measured at higher temperatures.