Surprisingly little is known about diffusion of even common gasses in polystyrene. The rate of transfer of blowing gents, for example, and the effect of gas exchange in foamed polystyrene cells and concomitant property changes depend on reliable prediction of diffusion through the bulk solid and its foamed counterparts. A simple diffusion apparatus was designed and experiments were conducted to provide data on gas diffusion in solid films. Various models were developed and used to interpret the experimental results for both steady and transient diffusion. Diffusion of Ar, He, N2 and various light hydrocarbon molecules was studied.