Thin films of polystyrene (PS) were modified using oxygen (0+) and helium (He+) ions and analyzed via diffusion and hardness measurements. One micron thick films were modified with 400 keV 0+ and He+ ions. The fluence was varied from 1.7 to 34.0 x 1013 ions/cm2 for both incident ions. Following irradiation, some samples were covered with a probe layer of deuterated PS, and annealed at 170°C. Using forward recoil spectrometry, the diffusion of d-PS and the PS hydrogen loss were measured as a function of incident ion species and dose. For all modified samples, d-PS diffusion was significantly retarded compared to the unmodified case. Atomic force microscopy measurements on the as-cast and irradiated PS samples suggest that the relative hardness of the 0+ modified PS was greater than that of the as-cast PS. The force-displacement curves of the modified PS displayed no hysteresis whereas the as-cast PS had a large hysteresis due to tip penetration and tip-polymer adhesion. These results indicate that ion irradiation decreases transport in PS and increases PS hardness.