The distribution, acid strength, and steam stability of internal and external acid sites were systematically investigated for nanocrystalline MFI zeolites with different crystallite size (2-300 nm), which were obtained using gemini-surfactants having different alkyl chain lengths. FT-IR spectroscopy using pyridine and 2,6-di-tert-butylpyridine as a base probe revealed that the concentration of external acid sites increased with a decreasing crystallite size. The external sites were weaker than the internal sites in acid strength, but could still catalyze n-octane cracking reactions that require the strongest acid strength among various acid-catalyzed reactions. Most notably, the external acid sites of zeolites exhibited remarkably higher steam stability at 873 K than the internal acid sites. Such an observation was supported by the catalytic results in methanol to dimethylether (DME) and gasoline conversion, and the Claisen-Schmidt condensation involving bulky molecules. (C) 2012 Elsevier Inc. All rights reserved.