A new model for characterizing the free volume of a glassy polymer { gas systems is proposed. An improved method for the calculation of occupied volume per monomer unit was developed within the limits of this model. The model assumptions, error estimates and algorithm e¯ciencies are described. Using the example of polyvinyltrimethylsilane, it is shown that linear dependences of logarithms of the di¬usion and the permeability coe¯cients on speci c accessible volume for inert gases exist.