We developed a model to describe the diffusion of gases through partially crystalline polymers. Diffusion is negligible in the crystalline fraction and occurs mainly in the amorphous region; however, crystallinity affects chain stiffness in the amorphous fraction. In our model this effect is taken into account in the activation energy calculation. The semicrystalline polymer in which diffusion takes place is treated as a heterogeneous medium in the framework of effective medium theories. An effective diffusion coefficient is obtained that takes into account the influence of both the presence of crystallites and the distribution of chain separations. We compare the results of two effective medium models and thus determine their respective applicability ranges.