[en] Complete text of publication follows. The reactions of C.I. Reactive Black 5 in aqueous solutions with the e_aq^-, OH radical and H atom intermediates of water radiolysis were investigated by pulse radiolysis. The transients formed with all the three intermediates have absorption maxima at ∼360 nm, ∼410 nm and ∼470 nm. The e_aq^- adds to the azo group and the adduct radical anion rapidly protonates forming hydrazil radical. Based on the similar spectra the same intermediates are produced in H atom and e_aq^- reaction. Due to the higher reactivity, the OH radical may react in several parallel processes: the most probable way is the OH radical addition to the azo group. Moreover, the OH radical can attack to the aromatic rings. The intermediate formed in the last reaction has a high absorbance at 360 nm due to the increased conjugation of the radical site with the aromatic rings. Another possible reason for the higher absorbance can be that the hydrazil radical may react with another dye molecule forming dimer radical (RB₂). The kinetic curves were measured in wide range of wavelengths and the rate coefficients were determined by fitting procedures. The buildup and the decay of the absorbance were found to follow first and simple second order behaviour, respectively