On γ-irradiation of solid cyclohepta-amylose, there is rupture of the cyclic α-1 → 4-glycosidic system which does not follow the pattern of acid hydrolysis. No glucose is produced, and the products formed initially are maltohexaose and gluconic acid. The gaseous products are hydrogen, carbon dioxide, and carbon monoxide, and, from linear yield–dose curves, G(H2)= 3·1, G(CO)= 0·9, and G(CO2)= 2·2. For dry cyclohepta-amylose –G is 14·7. Water, even in a molar ratio of 1 : 1, with cyclic dextrin stabilises the dextrin to radiation damage, and –G falls to 7·5. For the dry dextrin and monohydrate, stoicheiometric amounts of maltohexaose and acid are produced on irradiation. Further water up to a stoicheiometric ratio of 10 : 1 water to dextrin does not significantly alter the –G-value but reduces the amount of acid produced. The radiation degradation of cyclohexa-amylose follows a similar pattern. On irradiation of cyclohepta-amylose in aqueous solution (8 × 10–3 M), there is a different distribution of products, and cleavage of the cyclic system is more akin to acid hydrolysis.