A two-step swelling method was used for preparing porous poly(ionic liquid)s based on the copolymer of 1-allyl-3-methylimidazolium tetrafluoroborate and acrylonitrile P([AMIM]BF 4 -AN), the copolymer of 1-allyl-3-methylimidazolium hexafluorophosphate and acrylonitrile P([AMIM]PF 6 -AN), and poly(vinylbenzyl trimethylammonium) tetrafluoroborate P[VBTMA]BF 4 . The characteristics of the polymers were assessed via FT-IR spectroscopy, scanning electron microscopy (SEM) and CO 2 adsorption. The results indicated that the seed-swelling method was more effective in improving the CO 2 adsorption capacity of imidazolium-based P([AMIM]BF 4 -AN) containing the [BF 4 ] − anion. Exchanging [BF 4 ] − for [PF 6 ] − in the imidazolium-based poly(ionic liquid) led to almost the same CO 2 adsorption as exhibited by the corresponding non-swelling copolymer. Moreover, the CO 2 adsorption capacity of ammonium-based P[VBTMA]BF 4 when treated by the seed-swelling method decreased considerably.