The reactions of Br2˙−, Br˙, HO˙ and N3˙ with Aldrich humic acid (AHA) were investigated. The Br/Br2˙− radicals were obtained in flash-photolysis experiments (λexc = 266 nm) performed with NaS2O8 solutions in the presence of bromide ions. HO˙and N3˙ radicals were generated by pulse radiolysis of N2O-saturated solutions. From the combination of a bilinear analysis and computer simulations of the absorbance traces, it was possible to obtain information on the rate constants for the reactions of Br2˙−, Br˙, HO˙ and N3˙ with AHA and on the intermediate species involved in the mechanism. Evidence for the participation of phenoxyl radicals (λmax = 410 nm) is given.