Oxy-functionalized 1,2,4-trioxanes or 1,2,4-trioxepanes were produced by either autoxidation of the unsaturated hydroperoxy acetals 1, or the acid-catalyzed cyclization of epoxy hydroperoxides 9 which were readily prepared from 1. By the appropriate choice of cyclization procedure, the structures of the resulting cyclic peroxides could be efficiently controlled. (C) 1997 Elsevier Science Ltd.