Cross-linked membranes for gas separation have been prepared by thermal treatment of carboxylated polymers of intrinsic microporosity (C-PIMs). The optimal cross-linking temperature was investigated and possible cross-linking pathways involving aryl radical-induced thermal decarboxylation are provided, while several other possible mechanisms are ruled out. Carboxylated PIMs are accessible by controlled hydrolysis of the nitrile-containing parent polymer. The resulting cross-linked PIMs were insoluble in typical solvents and were characterized by Fourier transform infrared spectroscopy (FTIR), TGA-MS, TGA-FTIR, and gel content analysis. The decarboxylated PIM (DC-PIM) membranes showed higher selectivities for the O-2/N-2, CO2/N-2, and CO2/CH4 gas pairs, with evidence of suppression of swelling-induced densification under high CO2 pressure.