The mass transport properties of n-alkanes in pure poly(1-trimethylsilyi-1-propyne) (PTMSP) and in its copolymers with poly(1-trimethylsilyl-1-hexyne) (TMSE) have been investigated using gravimetric techniques. The results show in all cases a decrease in solubility and diffusivity as the fraction of TMSE increases; that indicates a significant effect of TMSE on the fractional free volume of the polymeric matrix. The evaluation of the mobility parameter from the diffusion data further confirms this effect. The behavior at different temperatures shows that the. sorption process is exothermic; the absolute value of the heat of mixing decreases with increasing the TMSE content, while the activation energy for diffusion increases. The effect of the penetrant molecular weight on solubility is examined in some detail and interpreted based on the NELF model.