A process for the preparation of a diacylated 4-imidazolin-2-one of the formula ##STR1## wherein R.sup.1 and R.sup.2 independently represent hydrogen, optionally substituted alkyl, cycloalkyl, aryl and alkoxy, which comprises contacting a compound of the formula ##STR2## wherein R.sup.1 and R.sup.2 have the previously assigned significance with a source of chlorine at a temperature from 20.degree. to 150.degree. C. in the presence of a radical forming agent or under ultra-violet light and thereafter dehydrohalogenating or dehalogenating the resultant chlorinated product.The diacylated 4-imidazolin-2-ones prepared by the process are useful for photochemical cycloadditions and as intermediates for the preparation of compounds in the biotin series.