In this study, diffusion behavior is investigated for polyethylene - polystyrene semi-interpenetrating polymer networks (PES-IPN) with toluene and chloroform and for a polyethylene/toluene system. The results show that diffusion in the semi-IPN material consists of a continuous polyethylene phase containing disperse polystyrene and that the transport in this material closely parallels transport in the homogeneous polyethylene. Consequently, the measured diffusion coefficients can be correlated, using the free-volume theory and parameters which characterize the free volume of the continuous phase, as a function of temperature and concentration. In addition, from the diffusivities of toluene in the polyethylene and the solubilities in the polyethylene and polystyrene, the diffusion coefficients of the PES-IPN/toluene system are predicted with an unsteady-state diffusion model developed in this study. Predictions with the new unsteady-state model are comparable with the experimental data.