Capillary column inverse gas chromatography has been used to determine the partition and diffusion coefficients of several solvents in a polymer blend at different temperatures, in order to study the effect of the concentration fluctuations present in the blend on the diffusivity of the penetrants through the blend. In this chapter, results are presented for a study of a polymer blend composed of polystyrene and poly(vinyl methyl ether) in three different blend compositions (pure PS, 75/25 PS/PVME, and pure PVME) using CCIGC. In this polymer blend, the difference of glass transition temperature between the two components is big (approx. 125^ºC) and the components present weak molecular interactions, which provide huge concentration fluctuations in the polymer matrix. It was found that in the blend, the diffusivity of the solvents was slightly larger that the expected linear-interpolated value (obtained by calculating the average of the diffusivity values of the two homopolymers). This leads one to think that the different nano-domains in the polymer blend have some effect on the macroscopic diffusivity of the solvent, acting as if the blend behaved as a porous membrane.