Laser flash photolysis of alpha-hydroxy and alpha-amino ketones, which are used as photoinitiators in free radical polymerization, lead to the generation of a series of nucleophilic alpha-hydroxy and alpha-amino radicals. Absolute addition rate constants of these radicals to n-butylacrylate and oxygen were measured by laser flash photolysis employing an indirect probe technique. Crystal violet and N,N'-bis(2,5-di-tert-butylphenyl)-3,4,9, 10-perylenedicarboximide were used as selective probe molecules for these nucleophilic initiator radicals to measure the addition rate constants to n-butylacrylate and oxygen, respectively. High acrylate addition rate constants of some initiator radicals were found in acetonitrile solution; e.g., dimethylketyl radical (k(acrylate) = 1.3 x 10(7) M-1 s(-1)) and 2-morpholino propan-2-yl radical (k(acrylate) = 2.9 x 10(7) M-1 s(-1)).