Four kinds of ZSM-5 zeolites with different SiO2/Al2O3 ratios are alkali-treated in 0.2 M NaOH solution for 300 min at 363 K. Changes to the compositions, morphologies, pore sizes, and distributions of the zeolites are compared before and after alkali-treatment. The changes observed are largely influenced by the SiO2/Al2O3 ratios with which the zeolites are synthesized. A possible mechanism of desilication during alkali-treatment is proposed. The SiO2/Al2O3 ratio of zeolites is found to influence the yield of light olefins that use heavy oil as feedstock. Alkali-treated ZSM-5 zeolites produce higher yields of light olefins compared to either untreated zeolites or the industry catalyst CEP-1. It is believed that alkali-treatment introduces mesopores to the zeolites and improves their catalytic cracking ability. ZSM-5 zeolites with SiO2/Al2O3 ratios of 50 also present superior selectivity toward light olefins because of their optimized hierarchical pores. (C) 2010 Elsevier B.V. All rights reserved.