Here, we report the transport properties of substituted polyphosphazenes membranes for separation and purification of CO2 for in-situ resource utilization (ISRU) processes. This paper includes pure and mixed gas permeation data of gases for the methoxy ethoxy ethanol (MEE)-substituted polyphosphazenes. Significant differences in transport properties of gases have been observed for the cross-linked and un-cross-linked polymer membranes. The CO2/N-2 ideal separation factors for the un-cross-linked and cross-linked membrane for 74% MEE polyphosphazene were 16 and 43 at 22 C, respectively. An effort has been made to relate these differences to polymer structure through apparent energy of activation calculations and observed glass transition temperatures. The CO2 permeability for the cross-linked 74% MEE polyphosphazene membrane decreased from 71 to 20 barrrers with an increase in the CO2/N-2 ideal separation factor from 43 to 110 as the membrane temperature is reduced from 22 to -20 degrees C. Also, experiments were performed under simulated Martian atmospheric conditions for the 48% MEE polyphosphazene at 80 and 800 mbar feed pressure. The permeability of all gases is independent of the feed pressure in the range 1500-3000 mbar for the 74% MEE polyphosphazene and at low pressures (80-800 mbar) for 48% MEE polyphosphazene. The variations in permeability of gases with temperature for both the 74% and the 48% MEE polyphosphazenes are also reported in this article.