The decomposition of 2,2’-azobisisobutyronitrile (AIBN) and di-methyl-Ar-(2-cyano-2-propyl)ketenimine (1) were studied using a radical scavenger, l,l,3,3-tetramethylisoindolin-2-yloxyl (2), to isolate the cage reaction. Anomalous results were obtained, and these are explained by an induced decomposition of the ketenimine (1) by the radical scavenger (2), which results in an increased rate of decomposition and a change in the ratio of combination:disproportionation products. A mechanism for the induced decomposition is proposed. © 1994, Taylor & Francis Group, LLC. All rights reserved.