[en] Complete text of publication follows. The degradation and decoloration of three textile (JGB, Janus Green B, RB5, Reactive Black 5 and AR, Apollofix Red) dyes by gamma irradiation have been studied. Colour and pH of the dye solutions were found to decrease with irradiation, The change of absorption spectra, the degree of decoloration, the change of pH, COD (chemical oxygen demand), BOD (biological oxygen demand) were measured as a function of irradiation dose, dose rate and dye concentration. In all cases the absorbance of the dye compounds was found to decrease with the increase of the dose. The effect of pH, N2, N2O on the degradation and decoloration process was studied. The degradation and decoloration in the presence of nitrous oxide occurred faster than in air and nitrogen-saturated solutions. The degree of decoloration for AR and RB5 increased with the increasing irradiation dose and dose rate but above 2 kGy/h dose rate, the degree of decoloration is independent of the dose rate and irradiation dose. The degree of decoloration increased up to 100% for the nitrous oxide-saturated RB5 solution from 80% for the nitrogen-saturated RB5 solution at 0.8 kGy irradiation dose and at 2 kGy/h dose rate. For AR aqueous solution in air and nitrogen-saturated and nitrous oxide-saturated AR solution, the degrees of decoloration were very similar to each other. Although the absorptions at maximum wavelength (596 nm) decreased with increasing irradiation dose for nitrogen-saturated RB5 aqueous solutions, for nitrous oxide-saturated RB5 aqueous solutions they decreased and shifted to lower wavelength. This suggests that the skeleton of the RB5 molecule is mainly destroyed by the attack of the OH radicals. AR and RB5 dyes are destroyed in the pH range from 7 to 4 with a reduced sensitivity at lower and higher pH range