T-5 and T-8 clusters (T = Si, Al) were used to calculate the structure of alkali cation-exchanged zeolites using DFT-based methods. The activity of basic zeolites was estimated from reactions with HCl and ethylchloride proton elimination. For the first case, all of the zeolite clusters reacted exothermically, with LiT5 presenting a DeltaH = -29.7 kcal/mol and CsT5 showing a DeltaH = -24.7 kcal/mol at the B3LYP/6-311+G** level of theory. For the proton elimination from ethylchloride to form ethene, only the LiT5 cluster had an exothermic reaction. The other clusters had a DeltaH varying from +0.1 kcal/mol for NaT5 to +2.3 kcal/mol for CsT5. The activation barriers for this reaction increased from Li to Cs. A careful analysis of the geometries, charges, and energetics for adsorption and activation barriers has shown that the reaction is controlled by the Lewis acid/base interaction between the cation and the chloride ion, not involving the Bronsted basicity of the framework oxygen atoms. These reactions proceed through a concerted mechanism that simultaneously involves the zeolitic basic framework oxygen atoms and the cation, which acts as a Lewis acid.