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The goal of this project is the development and characterization of synthetic membranes for the separation and purification of CO2 from the Martian atmosphere for in-situ resource utilization (ISRU) applications such as in-situ propellant production. Candidate materials should have high selectivity for carbon dioxide over nitrogen and argon, and a glass transition temperature of -40 °C or less to remain in rubbery state at low temperature for high permeance (flux/driving force). Membrane materials we identified include the rubbery polymers poly(dimethyl siloxane) (PDMS) and the copolymer poly(dimethyl, methylphenyl siloxane) (PMPS). Pure and mixed gas permeation experiments with CO2, N2 and Ar were performed with these membrane materials in the temperature range -25 to 21 °C. In experiments with the commercially available PDMS membranes, the pure gas CO2 permeability increases from 1932 Barrers to 2755 Barrers as the temperature decreases from 22 to -30 °C.