This study reports and analyzes gas transport and sorption properties between 35 and 50 degrees C for polymeric and carbon molecular sieve (CMS) membranes derived from an in-house synthesized polyimide referred to as 6FDA/DETDA: DABA(3: 2). The analysis of activation energies of permeation and diffusion and heats of sorption for CO2, CH4, O-2, and N-2 in these membrane materials provides insights regarding the permeability and selectivity changes at increasing pyrolysis temperature between 550 degrees C and 800 degrees C. The diffusion selectivity is factored further into "energetic" and "entropic" selectivity contributions to show that higher permselectivity of CMS membranes compared to polymeric membrane arises from a larger than unity entropic selectivity. CMS membranes pyrolyzed at 800 degrees C are also shown to have significantly elevated entropic selectivity compared with membranes pyrolyzed at 550 degrees C. Analysis of the selectivity factors provides fundamental insights into the importance of entropic factors as tools to tailor membrane performance.