Four diacylated pelargonidin (Pg: SOA-4 and SOA-6), cyanidin (Cy: YGM-3), and peonidin (Pn: YGM-6) 3-sophoroside-5-glucosides isolated from the red flowers of the morning glory, Pharbitis nil cv. Scarlett O'Hara (SOA), and the storage roots of purple sweet potato, Ipomoea batatas cv. Ayamurasaki (YGM), were subjected to an alpha-glucosidase (AGH) inhibitory assay, in which the assay was performed with the immobilized AGH (iAGH) system to mimic the membrane-bound AGH at the small intestine. As a result, the acylated anthocyanins showed strong maltase inhibitory activities with IC(50) values of <200 microM, whereas no sucrase inhibition was observed. Of these, SOA-4 [Pg 3-O-(2-O-(6-O-(E-3-O-(beta-D-glucopyranosyl)caffeyl)-beta-D-glucopyranosyl)-6-O-E-caffeyl-beta-D-glucopyranoside)-5-O-beta-D-glucopyranoside] possessed the most potent maltase inhibitory activity (IC(50) = 60 microM). As a result of a marked reduction of iAGH inhibitory activity by deacylating the anthocyanins, that is, Pg (or Cy or Pn) sophoroside-5-glucoside, acylation of anthocyanin with caffeic (Caf) or ferulic (Fer) acid was found to be important in the expression of iAGH (maltase) inhibition. In addition, the result that Pg-based anthocyanins showed the most potent maltase inhibition, with an IC(50) value of 4.6 mM, and the effect being in the descending order of potency of Pg > Pn/Cy strongly suggested that no replacement at the 3'(5')-position of the aglycon B-ring may be essential for inhibiting iAGH action.