A theoretical approach has been developed to describe the sorption and diffusion process of low weight molecular gases and vapors in polymers at wide ranges of sorbate concentration. The equation of an S-shaped gas sorption isotherm in glassy polymer matrix has been derived. The concentration dependence of the sorbate molecule diffusion coefficient has been established. For an S-shaped sorption isotherm, this dependence is nonmonotonous. The conditions of cluster formation of sorbate molecules have been analyzed within the proposed approach, in which it is possible to determine a correlation between these conditions and parameters of sorption isotherm. The comparison of the experimental and theoretical data provides an assessment microscopic characteristics of investigated polymer-vapor systems, such as the distances between vapor molecules in a matrix corresponding to intermolecular repulsion and attraction.