for the E isomer (E/Z ratios ranging from 90:10 to 97:3). The olefination reactions of aldehydes with dissymmetric arsonium halides (R not equal R') are very chemoselective; with arsonium ylide anions the benzyl moiety is more reactive than the allyl moiety which is much more reactive than prenyl and methyl groups. Based on the experimental results, a mechanism is proposed for the reaction.