α-Phenylselanyl aldehydes are prepared on a large scale by reaction of PhSeCl3 with the corresponding aldehydes in acetonitrile without isolation of the intermediate dichloro adducts. This method has been applied to α-phenylselanyl ketones derived from alkyl aryl ketones, symmetrical aliphatic ketones and alkyl isopropyl ketones. cis-4-tert-Butyl-2-phenylselanylcyclohexanone was also prepared in the same way.