The degraded products of iJ-cyclodextrin (iJ-CD) by r-irradiation in aqueous solution were identified as six oligosaccharides, such as glucose, maltose, maltotriose, maltotetraose, maltopentaose and maltohexaose, by the gel-filtration method on Biogel P-2 at 60°C. Analysis of the hyperfine structure of the ESR spectrum observed in the oxidation of iJ-CD with Ti3+hydrogen peroxide system indicated that a radical was formed mainly by hydrogen abstraction at C-5 of a glucose residue. A mechanism of the ring cleavage of iJ-CD by r-irradiation in oxygen-free aqueous solution was proposed.