6''-O-Succinyl-4'-hydroxyisoflavone-7-O-beta-D-glucopyranoside (succinyl-beta-daidzin) and 6''-O-succinyl-6,4'-dihydroxyisoflavone-7-O-beta-D-glucopyranoside (succinyl-beta-genistin), 2 new isoflavone metabolites, are found in cheonggukjang or natto, traditional soy-based foods fermented with Bacillus species. Standard isoflavones including daidzin, genistin, daidzein, and genistein, and mixtures of isoflavones extracted from roasted soybeans were added to the medium growing Bacillus subtilis or B. subtilis natto and formation of succinyl-beta-daidzin and succinyl-beta-genistin were monitored by high-performance liquid chromatography (HPLC). Samples containing Bacillus with daidzin and genistin produced succinyl-beta-daidzin and succinyl-beta-genistin, respectively, while those with daidzein and genistein did not produce succinyl derivatives. Daidzin in samples with B. subtilis and B. subtilis natto decreased by 39.7% and 10.7%, respectively, for 4 h incubation while genistin decreased by 66.8% and 17.6%, respectively. Genistein decreased faster than daidzein during incubation with B. subtilis or B. subtilis natto without formation of succinyl derivatives. In the case of mixture of isoflavones, succinyl derivatives increased and beta-glucosides and aglycones of isoflavones decreased significantly for 8 h incubation (P < 0.05).