Several UV absorbing chromophores have been incorporated in ammonium triphenylbutyl- and tetraphenylborates as light-harvesting groups, and the reactivity of the resulting salts as unimolecular free radical initiators has been studied. The results are compared with kinetic and thermodynamic data calculated for the primary photochemical step leading to generation of the initiating species. The dependence of the initiation reactivities on the structure of both the chromophore and the borate counteranion is discussed.