Enhanced catalytic performance for C-4 olefin cracking to propene and ethene was observed on phosphorus-modified HZSM-5 treated in steam at 1073 K. Combining the results of P-31 and Al-27 MAS NMR, D-2/OH exchange, NH3-TPD, and catalytic performance of the catalysts, it seems reasonable to conclude that new and hydrothernially stable acid sites are formed during the steam treatment through the interaction of phosphorus species with silanol nests exposed on the zeolite due to dealumination. It is tentatively suggested that the incorporated phosphorous species stabilized by some nonframework aluminum species are responsible for both the enhanced acid stability and the significantly improved catalytic performance for the cracking of C-4 olefin. (C) 2007 Elsevier Inc. All rights reserved.