The reactions of hydroxyl radical (OH) with guanidine, 1,1-dimethylguanidine,and 1-ethylguanidine have been investigated by the method of pulse radiolysis. The characteristic absorption observed in the OH reaction with guanidine under basic conditions was assigned to a nitrogen-centered radical. On the other hand, reducing carbon-centered radicals were generated in the OH reaction with 1,1-dimethylguanidine and 1-ethylguanidine over a pH range of 7-13, which may suggest that the primarily formed nitrogen-centered radicals at a high pH are converted into carbon-centered radicals by hydrogen atom shift. Similar transient species were also observed in the reaction of sulfate radical anion with the guanidine derivatives. (C) 2008 Elsevier B. V. All rights reserved.