The mechanism and diastereoselectivity of cyclopropanation reactions of arsenic ylides with enones have been investigated using density functional theory (DFT) calculations. The reactions of two different arsenic ylides, semistabilized (Ph) and stabilized (COMe), with enone have been considered. Calculations reveal that the transoid mode of addition is more favoured than the cisoid mode of addition in both reactions. Steric interactions induced by ylidic substituents and the methyl group of enones, and weak C–H⋯O hydrogen bond interactions are mainly responsible for the inherent stability difference between the addition transition states. The initial addition is found to be the diastereoselectivity-determining step for both semistabilized and stabilized ylides. The slight preferences for the formation of cis-cyclopropanes over trans-cyclopropanes in both cases is explained on the basis of weak H-bonding interactions and steric interactions encumbered on the diastereomeric transition states.