Carbon membranes are a potentially attractive candidate for the in-situ removal of water vapor in CO₂ hydrogenation reactions. Their hydrophilicity and pore structure can be tuned by properly adjusting the synthesis procedure. Herein, we assess the effect of the carbonization temperature (450–750 °C) on the performance of supported CMSM in terms of vapor/gas separation, in correlation with changes in their surface functionality and porous structure. FTIR spectra showed that the nature of the functional groups changes with the evolution of the carbonization step, leading to a gradual loss in hydrophilicity (i.e., OH stretching disappears at T_carb ≥ 600 °C). The extent of water adsorption displays an optimum at T_carb of 500 °C, with the membrane carbonized at 650 °C being the least hydrophilic. We found that the pore size distribution strongly influences the water permeance. At all T_carb, adsorption-diffusion (AD) is the dominant transport mechanisms. However, as soon as ultra-micropores appear (T_carb: 600–700 °C) molecular sieving (MS) contributes to an increase in the water permeance, despites a loss in hydrophilicity. At T_carb ≥ 750 °C, MS pores disappear, causing a drop in the water permeance. Finally, the permeance of different gases (N₂, H₂, CO, CO₂) is mostly affected by the pore size distribution, with MS being the dominant mechanism over the AD, except for CO₂. However, the extent and mechanism of gas permeation drastically change as a function of the water content in the feed, indicating that gas/vapor molecules need to compete to access the pores of the membranes.