Reported here for the first time are the generation of monocarbonyliodonium ylides and their alkylidene-transfer reactions to aldehydes yielding alpha,beta-epoxy ketones. Exposure of (Z)-(2-acetoxy-1-decenyl)iodonium bromide, prepared stereoselectively by sodium acetate-catalyzed Michael addition of acetic acid to (1-decynyl) (phenyl)iodonium salt, to EtOLi in THF at -78 degrees C results-in ester exchange to generate the monocarbonyliodonium ylide with the liberation of ethyl acetate. H-1 NMR measurements indicate that the ylide is stable up to -30 degrees C in THF-d(8) but gradually decomposes at -20 degrees C to 1-bromo-2-decanone. The monocarbonyliodonium ylide acts as an alkylidene-transfer agent to carbonyl compounds, and the reaction with aldehydes in THF-DMSO at -30 degrees C gives alpha,beta-epoxy ketones with E-isomers as a major product. With an alpha,beta-unsaturated aldehyde, selective 1,2-addition to the carbonyl group was observed, The relative rates of the alkylidene-transfer reaction of this ylide for a series of ring-substituted benzaldehydes were measured. A Hammett correlation plot with the a constants of substituents afforded the reaction constant: rho = 2.95 (r = 1.00), which indicates that the monocarbonyliodonium ylide is moderately nucleophilic in nature.