Aqueous solutions of reactive blue 4 textile dye are totally mineralized when irradiated with TiO₂ photocatalyst. A solution containing 4 x 10-⁴M dye was completely degraded in 24 h irradiation time. The results showed that the dye molecules were completely degraded to CO₂, SO₄, NO₃, NH₄and H₂O under solar irradiation. The addition of hydrogen peroxide and potassium persulphate influenced the photodegradation efficiency. The rapidity of photodegradation of dye intermediates were observed in the presence of hydrogen peroxide than in its absence. The auxiliary chemicals such as sodium carbonate and sodium chloride substantially affected the photodegradation efficiency. High performance liquid chromatography and chemical oxygen demand were used to study the mineralisation and degradation of the dye respectively. It is concluded that solar light induced degradation of textile dye in wastewater is a viable technique for wastewater treatment. ⓒ 2002 Elsevier Science Ltd. All rights reserved.