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Szlnwaary. Transient alkyl and acyl radicals are detected and identified by electron spin resonance during UV.-Irradiation of dialkyl and alkyl aryl ketones in liquid solutions. They are formed by thc primary processes of a-cleavage, intermolecular photoreduction or a-chloro elimination. For sevcral ketones a temperature dependent competition between cr-cleavage and photoreduction is observed indicating a higher energy of activation for the cleavage than for the reduction process. The results are discussed with respect to the photochemical behaviour of the ketones.Electron spin resonance parameters are reported for a variety of radicals. In particular 13C coupling constants support a sigma type structure of acyl radicals and require a nearly planar radical ccnter of t-butyl and isopropyl radicals.1 . Einleitung. -Das photochemische Verhalten vieler Ketone in Losung ist in den letzten Jahren eingehend untersucht und auf das Eintreten einer oder mehrerer der folgenden Primarreaktionen nach Anregung des nn*-ubergangs der Carbonylp p p e n zuruckgefuhrt worden [I41 :a-Spaltung (Typ I) hv RI-CO-R,Rl-CO-( CH,) ,-CHR,R,RI-CO-CHC1-R,