The structure of Li+, Na+ and K+-exchanged faujasite (FAU) zeolite is studied by means of cluster model and QM/MM approach at HF/6-31G** level of calculations. When CO molecule interacts with cation, "hard" site present in the zeolite channel, the CO bond length decreases compared to the free CO molecule and this decrease in CO bond length depends on Lewis acid strength of the cation. The strength of Lewis acidity of the cations and basicity of framework oxygen atoms is investigated using density functional (DF) methods. We used local reactivity descriptors based on hard-soft acid-base concept as acidity and basicity parameters. The calculated 'relative electrophilicity' and 'relative nucleophilicity' values are found to be better descriptors of acidic and basic sites, respectively. From relative electrophilicity (s(k)(+)/s(k)(-)) value the Lewis acidity of the cation-exchanged faujasite zeolite is found to decrease in the order: [Li-FAU] > [Na-FAU] > [K-FAU], and from relative nucleophilicity (s(k)(-)/s(k)(+)) value of the framework oxygen atom, the Lewis basicity is found to increase in the order: [Li-FAU] < [Na-FAU] < [K-FAU]. (C) 2002 Elsevier Science B.V. All rights reserved.