A molecular model for diffusion of gases and vapors of low-molecular weight at low pressures through semicrystalline polymer films is developed. The formulation of a molecular model allows us to consider the temperature dependence at constant penetrant concentration of the effective diffusion coefficient. Crystallinity influence is considered through the use of the generalized "chain immobilization factor" (Part I). We analyze the shapes adopted by polymeric chains allowing a diffusive step, and we obtain an expression for the activation energy E(D), considering both symmetric and nonsymmetric chain separations. The latter is centrally important in gases and vapors diffusion in semicrystalline polymers. Finally, we compare the results of our model with experimental data drawn from bibliography, obtaining a good agreement.