Cyclopropyl malonoyl peroxide (1), which can be prepared in a single step from the commercially available diacid, is an effective reagent for the dihydroxylation of alkenes. Reaction of 1 with an alkene in the presence of 1 equiv of water at 40 degrees C followed by alkaline hydrolysis leads to the corresponding diol (40-93%). With 1,2-disubstituted alkenes, the reaction proceeds with syn selectivity (3:1 to > 50:1). A mechanism consistent with the experimental findings that is supported by oxygen-labeling studies is proposed.