2'-Carbonyl 5-S-acetyl-C-glycofuranosides and 2'-carbonyl 4-S-acetyl-C-glycopyranosides were converted in good yields to respective 5-thio-C-glycopyranosides and 4-thio-C-glycofuranosides under base treatment. The transformation was resulted from beta-elimination on 2'-carbonyl C-glycoside to form alpha,beta-conjugated aldehyde (or ketone) and following intramolecular hetero-Michael addition by the thiol group. Crown Copyright (C) 2003 Published by Elsevier Science Ltd. All rights reserved.