Considerable interest had been focused during 1950-1980 on the pure gas isothermal sorption diffusion modeling in glassy polymers. In particular, the theory of dual-mode sorption has been developed during this period. It postulates that two concurrent sorption modes occur in the microheterogeneous medium. On the basis of this theory, rather complete and successful models have been roposed. In this paper, a short review of dual-mode-sorption models is first given. By considering a general expression of the sorption mode, the conventional models (partial immobilization model and immobilization model) become one limit-behavior model: the time-lag upper-bound model. Another limit-behavior model, the time-lag lower-bound model called the dual-diffusion model, is particularly investigated and compared to conventional models in this paper. It is shown that the steady-state permeability is independent of the molecule exchange rate, while the time-lag prediction depends on it. Furthermore, the time-lag lower-bound model provides, in general, a better agreement with experiments. In this respect, the dual-diffusion model is considered as a simple and efficient dual-mode-sorption theory approach.