The oxidation of 5-hydroxyalkenes by hydrogen peroxide, when catalyzed by CH3ReO3 (MTO), leads to functionalized tetrahydrofurans. In these cases, no tetrahydropyran was formed. The results show that the first reaction between the substrate and a peroxorhenium species formed from, and in equilibrium with, MTO and peroxide, yields an epoxide. This intermediate was not detected, however, because cyclization occurred so rapidly. 6-Hydroxyalkenes were similarly but more slowly converted to tetrahydropyran alcohols. A 4-hydroxyalkene was epoxidized but not cyclized.