Epoxy resins are often applied to prevent sensitive electronics to come into contact with solvents. Therefore, it is necessary to know the diffusion through the epoxy resins to determine the thickness of the epoxy resins layer for different applications. In this work a diffusion model is developed on the basis of the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state. To consider the polymer network, PC-SAFT is combined with an elastic term. The driving force for the diffusion is the gradient of the chemical potential, but additionally the viscoelastic polymer relaxation below the glass transition temperature is considered. The model is applied to calculate the diffusion of water, isopropyl alcohol, and heptane through the epoxy resin at different temperatures. The modeled diffusion processes are in good agreement with experimentally determined data. Even highly anomalous absorption kinetics, which were measured for isopropyl alcohol diffusion, could be explained and calculated on the basis of the developed diffusion model.