A series of aliphatic diselenides and selenides containing coordinating substituents was tested for glulathione peroxidase (GPx)-like catalytic activity in a model system in which the reduction of tert-butyl hydroperoxide with benzyl thiol to afford dibenzyl disulfide and tert-butyl alcohol was performed under standard conditions and monitored by HPLC. Although the diselenicles showed generally poor catalytic activity, allyl selenides proved more effective. In particular, allyl 3-hydroxypropyl selenide (25) rapidly generated 1,2-oxaselenolane Se-oxide (31) in situ by a series of oxidation and [2,3]sigmatropic rearrangement steps. The remarkably active cyclic seleninate 31 proved to be the true catalyst, reacting with the thiol via a postulated mechanism in which the thioseleninate 32 is first produced, followed by further thiolysis to selenenic acid 33 and oxidation-dehydration to regenerate 31. In contrast to catalysis with GPx, formation of the corresponding selenenyl sulfide 34 comprises a competing deactivation pathway in the catalytic cycle of 31, as a separate experiment revealed that authentic 34 was a much less effective catalyst than 31. 1,2-Oxaselenane Se-oxide (37), the six-membered homologue of 31, was formed similarly from allyl 4-hydroxybutyl selenide (26), but proved a less effective catyalyst than 31. Compounds 31 and 37 are the first examples of unsubstituted monocyclic seleninate esters.