A recently developed spectroscopic technique was used to determine oxygen diffusion coefficients, D, as a function of temperature in polystyrene and polycarbonate films. The data were quantified by using an Arrhenius expression with the following variables : (a) a diffusion activation barrier, E(act), and (b) a diffusion coefficient, D-0, that represents the condition of "barrier-free" gas transport. From the perspective that diffusion depends on free volume, the parameters E(act) and D-0 are interpreted to reflect dynamic and static elements of free volume in the polymer matrix. The addition of low molecular weight solutes to these amorphous glasses can alter the oxygen diffusion coefficient by affecting the dynamic and/or static free volume of the material. These effects are manifested in the parameters E(act) and D-0. Depending on the temperature, the additive may either inhibit or facilitate oxygen diffusion relative to diffusion in the unperturbed, additive-free material. Data are reported for films containing dimethyl phthalate, diphenyl phthalate, cholestane, and molecular nitrogen as additives.