The adsorption Of CO2 in the alkali exchanged Y-faujasite type zeolite (Li+, Na+, K+ and Cs+) has been investigated using density functional theory calculations at the PW91 level. A cluster centered around the SII cation was cut from the periodic faujasite structure including a whole zeolite supercage. The computed geometric parameters of the bare clusters are compared with those of the crystal structure obtained by X-ray or neutron diffraction. The cation-CO2 geometry is then investigated as a function of the nature of the alkali cations. The calculated adsorption enthalpies show a decrease from Li+ to Cs+ and reproduce well experimental values obtained by microcalorimetry measurements. (c) 2006 Elsevier B.V. All rights reserved.