Abstract The CO2/CH4 separation performance under humidified mixed feed conditions relevant to biogas separation is reported for supported, epoxy-amine-based ion gel membranes containing fixed-site amine facilitated CO2 transport carriers. The chemical composition of the ion gel membranes consists of combination of the bis(epoxide)-IL monomer and trifunctional amine monomer in a mole ratio 3:2 plus either 50 or 75 wt% free [EMIM][Tf2N], impregnated into a Omnipore™ support film. Prepared samples were examined for fundamental structure/property relationships via permeation and sorption methods. Gas sorption confirmed specific gas interactions, showing elevated CO2 sorption compared to CH4 with increasing equilibrium feed pressure. Single gas permeation demonstrated almost a three-fold increase in CO2 permeability from 195 barrer for 50 wt% of free [EMIM][Tf2N] to 525 barrer for 75 wt% of ionic liquid while the ideal selectivity α(CO2/CH4) stayed almost the same (20 and 18, respectively). The effects of feed composition, feed pressure, and relative humidity (32% and 54%) on the CO2/CH4 separation performance were elucidated for mixed-gas feeds. Under simulated biogas processing conditions, an increase of CO2/CH4 separation factor from 25 to 35 with increasing humidity and low feed pressure was observed. Such behavior indicates that the fixed-site-carrier facilitated CO2 transport mechanism enhances also the CO2/CH4 separation performance of studied membranes, as observed for the CO2/N2 mixtures studied previously. This feature also enables them to reach a performance level close to the 2008 Robeson plot upper bound.