Hydrogels were successfully synthesized utilizing CO2 as a gellant. A cross-linking reaction of polyallylamine (PAA) with CO2 in the presence of 1,8-Diazabicyclo[5,4,0]-undec-7-ene (DBU) provided hydrogels bearing urea cross-linking points and residual amino groups in the side chains. The obtained hydrogels absorbed CO2 at 25 degrees C and gave a maximum absorption four times larger than that of PAA aqueous solution and 2.8 times larger than that of the most commonly used absorbent, monoethanolamine. The PAA hydrogels desorbed the absorbed CO2 completely under a N-2 atmosphere at 120 degrees C, and could be repeatedly recycled without loss of efficiency, indicating their potential application as recyclable CO2 absorption materials.