The reaction of methyl ketones with catalytic amounts of diphenyl diselenide and an excess of ammonium peroxydisulfate in methanol proceeds smoothly to afford a-keto acetals in good yield. In some cases reaction yields were increased by using stoichiometric amounts of PhSeSePh. It is suggested that the oxidation of PhSeSePh by (NH4)2S208 produces a very electrophilic phenylselenenylating agent that effects the methoxyselenenylation of the methyl ketones. The addition products so formed react with (NH4)2S208 to afford the methoxyde-selenenylation products; the phenylselenenylating agent is thus regenerated. Thus, the entire process can be effected in one pot and with catalytic amounts of PhSeSePh. Some examples of the intramolecular version of this reaction are also reported. © 1990, American Chemical Society. All rights reserved.