Concentration profiles and diffusion coefficients of deuterated polystyrene chains (27 000–104 000 amu) diffusing into hydrogenated polystyrene (h‐PS) matrices (90 000 and 590 000 amu) were measured by forward recoil of 2.0 MeV He ions. This experimental procedure allows us to detect the diffusion coefficient in the range 3×10−15–3×10−13cm2/s. The irradiation of theh‐PS layer with ion beam changes the interdiffusion properties. The diffusion coefficient, measured inh‐PS irradiated with 300 keV H+in the 5×1011–2×1014ions/cm2fluence range, does not change at low fluence, instead it is reduced for a threshold fluence whose value depends on the matrix molecular weight. High fluence irradiation ofh‐PS decreases the polymer solubility because crosslinks between the original chains, produced by ion irradiation, yield a three‐dimensional network (gel). By combining diffusion and solubility measurements, for both molecular weights, we observed that the diffusion process is slowed down when, in the irradiatedh‐PS is produced a gel fraction of 0.8, which corresponds to a polymer structure containing an average of 1.0 crosslink per chain. This result does not depend on the marker molecular weight.