A series of partially pyrolyzed membranes (PPMs) were prepared from a copolyimide having carboxylic acid pendant groups (PIA). The effect of the final pyrolysis temperature (425, 450 and 475 degrees C) on the gas separation properties was investigated and compared with those of PIA. The thermal treatments caused a progressive removal of the carboxylic acid pendant groups of PIA and a simultaneous increase of the fractional free volume of the PPMs. Thus, it was found that the permeability coefficients of PPMs were higher than that of PIA, between 6.5 and 25 times higher, depending on the gas tested and the final pyrolysis temperature, while the selectivity slightly decreased. All PPMs were attractive materials to separate CO(2) from CH(4) since were inside the commercially interesting region and are close to the present Robeson trade-off limit line. The pyrolysis at 475 degrees C yielded the PPM with the best performance: CO(2)/CH(4) selectivity of 47 and CO(2) permeability close to 95 barters. (C) 2009 Elsevier B.V. All rights reserved.