Lewis acidity of alkali cation-exchanged zeolite is studied using local reactivity descriptors based on hard-soft acid-base (HSAB) concept. The local softness for nucleophilic attack (s(x)(+)), local softness for electrophilic attack (s(x)(-)) and their ratio, which is called,relative electrophilicity' (s(x)(+)/s(x)(-)), are calculated for the exchanged cations and Lewis acidity of the cations is found to decrease in the order: Li+ > Na+ > K+ > Rb+ > Cs+. Calculated blue shift of CO vibrational frequency (Deltav) and interaction energy of CO molecule with alkali cation-exchanged zeolite clusters vary linearly with s(x)(+)/s(x)(-) values. (C) 2004 Elsevier B.V. All rights reserved.