Abstract In membrane based gas separation, crosslinking of membrane materials was shown to be an effective method to stabilize the polymer structure against undesirable plasticization effects. In this study a crosslinkable copolyimide was synthesized by using the dianhydride 6FDA (4,4′-(hexafluoroisopropylidene)diphthalic anhydride) and the diamines ODA (4,4′-oxydianiline), 4MPD (2,3,5,6-tetramethyl-1,4-phenylene diamine) and DABA (3,5-diamino benzoic acid). The last mentioned diamine contains a free carboxylic acid group where crosslinking of the polymer chains is possible by esterfication reaction with a diol. In the current work EG (ethylene glycol) was chosen as crosslinker. The esterfication conditions were varied to optimize the solid-state crosslinking reaction. To investigate the influence of crosslinking conditions on the separation properties of the membranes pure gas permeation experiments were performed with non-crosslinked and the different EG-crosslinked films at 35°C for the gases CO 2 using feed pressures up to 40 bar and CH 4 at pressures up to 10 bar.