The aerobic oxidative cleavage of a-methylstyrene and other benzylic olefins catalyzed by N-hydroxyphthalimide (NHPI) under solvent-free conditions is described. The reaction was remarkably accelerated by the addition of azo-initiator (2,2 0 -azobis(2-methylpropionitryl)) or transition metal salts (Mn, Cu, Co) in combination with NHPI. Alkylammonium salts and acetophenone were successfully applied to increase the solubility of the polar NHPI catalyst in non-polar substrates. The effects of the catalyst and reaction parameters on the conversion of the substrate and the selectivity for ketones were investigated. New insights into the oxidation mechanism and NHPI-ammonium salt interaction are presented.