Polystyrene (PS) foams are widespread heat insulation and impact absorption materials, but many blowing agents used in their production are harmful to the environment. Pentane is a less harmful alternative and is commonly used in the manufacturing of expandable PS, but there are inadequate diffusion data of pentane in PS available in the literature at conditions relevant to PS impregnation and foaming. We conducted gravimetric measurements of sorption dynamics of n-pentane in PS particles and thin films at temperatures from 45 to 110 degrees C and pressures from 0 to 4.8 bar. The measured data were fitted using the Fick's and case II diffusion models and the obtained transport parameters are presented together with an analysis of the dominant diffusion mechanism. We also present the Flory-Huggins interaction parameter for the PS + n-pentane system and show that it can be used to accurately predict the sorption equilibria. This paper thus provides important parameters for the optimization of PS foam production.