1-beta-Telluroglycosides isomerize to a-isomers upon photolysis and thermolysis. The isomerization follows first-order reaction kinetics and proceeds through a glycosyl radical intermediate by homolytic C-Te bond cleavage. The radicals so generated are trapped intermolecularly by TEMPO free radical, indicating that the glycosyl radicals are efficiently generated by a non-chain mechanism. (C) 1999 Elsevier Science Ltd. All rights reserved.