The inclusion complexation behavior of azadirachtin with several cyclodextrins and their methylated derivatives has been investigated in both solution and the solid state by means of XRD, TG-DTA, DSC, NMR, and UV-vis spectroscopy. The results show that the water solubility of azadirachtin was obviously increased after resulting inclusion complex with cyclodextrins. Typically, P-cyclodextrin (P-CD), dimethyl-p-cyclodextrin (DM beta CD), permethyl-beta-cyclodextrin (TM beta CD), and hydroxypropyl-beta-cyclodextrin (HP beta CD) are found to be able to solubilize azadirachtin to high levels up to 2.7, 1.3, 3.5, and 1.6 mg/mL (calculated as azadirachtin), respectively. This satisfactory water solubility and high thermal stability of the cyclodextrin-azadirachtin complexes, will be potentially useful for their application as herbal medicine or healthcare products. (c) 2005 Elsevier Ltd. All rights reserved.