The decolourisation of aqueous solutions of dyes by ozonation in the presence of hydrogen peroxide, potassium permanganate or Ferral using a bubble column reactor was investigated. Addition of hydrogen peroxide in the ozonation reaction of dyes did not affect its decolourisation efficiency. The addition of potassium permanganate considerably increased the decolourisation efficiency during ozonation of all of the dyes studied. Ferral showed excellent catalytic activity under acidic conditions but in alkaline conditions its catalytic activity diminished. The effect of the ozonation on COD was also monitored and was found to be highly effective in reducing the COD of aqueous dye solutions. In addition to studying the effect of addition of hydrogen peroxide, potassium permanganate or Ferral on the ozonation of aqueous solutions of individual dyes, tests were carried out on effluent from a commercial dyeing unit. (c) 2005 Society of Chemical Industry.