The enzymatic modification of genistin to enhance its water solubility was studied using two glycosyltransferases, cyclodextrin glucanotransferase from alkalophilic Bacillus sp. I-5 and 4-alpha-glucanotransferase from Thermus scotoductus. Two different catalytic reactions, the transglycosylation and cyclization activities, were observed when the reaction was performed with soluble starch as a donor and genistin as an acceptor. The reaction products were isolated and identified as [Glc(alpha1-4)](1-22)-Glc(beta1-7)-genisteins and cycloamylose with DP 8-12 by HPLC and MALDI-TOF MS. A beta-amylase treatment revealed inclusion complexes composed of Glc(alpha1-4)-Glc(beta1-7)-genistein/Glc(alpha1-4)-Glc(alpha1-4)-Glc(beta1-7)-genistein and cycloamylose with DP 8-12. The results indicated that the cycloamylose formed by the cyclization reaction of the enzyme included Glc(alpha1-4)-Glc(beta1-7)-genistein/Glc(alpha1-4)-Glc(alpha1-4)-Glc(beta1-7)-genistein. The presence of cycloamylopectin, in which the Glc(alpha1-4)-Glc(beta1-7)-genistein/Glc(alpha1-4)-Glc(alpha1-4)-Glc(beta1-7)-genistein was enclosed, was also observed with HPLC, HPSEC-MALLS, and MALDI-TOF MS analyses. The solubility of genistin was highly improved, and the solution containing glycosylated genistin and the inclusion complex demonstrated excellent properties of transparency and stability during storage at 4 degrees C.