The selectivity of radical generation in the oxidation of carbonyl compounds by manganese(III) has been studied in two ways. Competitive reaction of acetic and propionic acids under conditions in which the resultant radicals are trapped efficiently by an alkene suggests that propionic acid reacts some 3.8 times more readily than does acetic acid. Oxidation of butanone under similar conditions gives products in the formation of which reaction has occurred 2.9 times more readily at C-3 of butanone than at C-1. However, this ratio reflects a combination of chain propagation and of initiation by manganese(III) oxidation. When propagation is effectively eliminated, the C-1 position is the more reactive. The significance of these observations on the mechanism of oxidation of carbonyl compounds by manganese(III) is discussed. It is suggested that enol radical-cations can be formed in the reaction but that these do not give carbonyl-stabilised radicals by deprotonation.