The reactions of the C.I. Reactive Black 5 with short lived e(aq)(-), OH radical and H atom intermediates of water radiolysis in aqueous solution were investigated by pulse radiolysis with kinetic spectroscopic detection. The transients formed with the three intermediates have absorption maxima at similar to360, similar to410 and similar to470nm. The e(aq)(-) adds to the azo group and the adduct radical anion rapidly protonates forming hydrazil radical. Based on the similar spectra similar intermediates are produced in H atom and hydrated electron reaction. The OH radical most probably adds to the azo group, however, it attacks also the aromatic rings. The strong absorbance found is attributed to the high conjugation of the radical site with the aromatic rings. (C) 2003 Elsevier Science Ltd. All rights reserved.