Interconversion reactions of cyclodextrin glycosyltransferase (CGTase) among cyclodextrin (CD) homologues were experimentally investigated using each CD as a substrate in an aqueous, two-phase-forming polymer solution of dextran and polyethylene glycol. Degradation rate of alpha-CD was highest and that of beta-CD was lowest among alpha-, beta- and gamma-CD with Bacillus macerans CGTase. Degradation of each CD was accelerated with dextran, while decelerated with polyethylene glycol.