Diffusion coefficients for expanded polystyrene foam (EPS) and polystyrene film are vital information to many industries worldwide. The method used to obtain the data in this study was a transmission flow method. The diffusion data obtained during this study were on the same order of magnitude as previous researchers, and was rather consistent. The diffusion coefficients were obtained for argon gas through 0.08, 0.11 and 0.27 mm thick films, and helium gas through 0.11 mm thick film. The diffusion coefficients for argon through the 0.08 mm thick polystyrene film were between 2.07 and 2.21 x 10-11 m2/s , through 0.11 mm thick film were between 5.27 and 7.24 x 10-12 m2/s, through 0.27 mm thick film were between 4.91 and 6.22 x 10-12 m2/s, and the diffusion coefficients for helium through 0.11 mm thick film were between 5.1 and 6.08 x 10-11 m2/s. Some work was also done with diffusion through EPS, and the diffusion coefficients were on the order of magnitude of 10-7 m2/s.