The photochemical degradation of some selected monoand di– chlorotriazine dyes – C. I. Reactive Orange 13, C. I. Reactive Orange 4, C. I. Reactive Yellow 2, C. I. Reactive Blue 4, C. I. Reactive Blue 5 – and starting compounds used for various applications to cotton are examined. The simple, exponential relationship between relative number of decomposed molecules and absorbed energy is verified. It is found that the structure of the base (chromophore)influences the tendency of a dye to exhibit photodegradation. The bases improve in fastness to light in the following order: base of CI. Reactive Orange 4 and Orange 13>base of CI. Reactive Yellow 2>base of C. I. Reactive Blue 4 and Blue 5. By linking the base to the reactive part of the molecule, an increase in resistance to photochemical degradation is brought about, in dependent of the number of reactive chlorine atoms. A covalent bond between the dye and the fibre may, in some cases (C. I. Reactive Yellow 2), cause a decrease in photodegrad