Membrane permeation and separation characteristics of mixed gas/membrane systems are typically calculated from single-component transport parameters, namely, diffusion coefficients and solubility constants. In certain gas systems involving gaseous or vapor mixtures, where mass transport is affected by coupling effects or the competition of penetrants for unrelaxed free volume, such calculations can lead to erroneous estimates of the membrane separation efficiency. In this paper, we discuss the design and development of an experimental setup for observing mixed gas permeation through non-porous membranes. The novelty of the setup is the unique inline sampling interface which allows injection of permeate mixed gas samples for concentration analysis without introducing any leaks into the permeate volume. Also, a data cropping technique is introduced to elucidate the transport properties of gases through membranes under mixed gas permeation conditions. The method is employed in mixed gas permeation studies of a rubbery polymer, (PDMS), and a glassy polymer, (NEW-TPI), membranes, using different feed compositions of a CO2/CH4 binary gas system. The results show an absence of synergistic effects in gas permeation through the rubbery polymer, however, in the glassy polymer the transport parameters of each of the gases are highly affected by the presence of the other gas. Also, a change in the feed gas composition alters the separation properties of the glassy polymer.