By CO2 bubbling into an aqueous dimethyl sulfoxide solution, the polyamidine (Poly-Amd) prepared from hexamethylenediamine and triethyl orthoacetate was converted to the bicarbonate salt, Poly-AmdHHCO(3), which formed aggregates. Conversely, the aggregates disappeared upon Ar bubbling to release the captured CO2, reverting to Poly-Amd completely. Polyethyleneimine (PEI) also reversibly captured and released CO2 molecules in solution by bubbling CO2 and Ar, respectively, in an alternate manner. No appreciable difference was observed between Poly-Amd and PEI in CO2 capture in solution. The binary system consisting of Poly-Amd and polyethylene glycol (PEG) captured CO2 efficiently at ordinary pressure and reached a stationary state within 200 h, at which 66% of the amidine groups bound CO2 molecules, which was released upon exposure to a N-2 flow. In contrast to the binary system with Poly-Amd, the binary system of PEI with PEG did not capture CO2 efficiently, and only 5.7% of the amino groups bound CO2 molecules after 600 h. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3404-3411