Different dyes can exhibit markedly different reaction rates with ozone. These observations can raise the concern that in a mixture of dyes one of them can be over-oxidized while the other remains under-oxidized, i.e. exhibiting colour. In addition, the effect of pH on decolourisation also deserves additional investigation. The influence of pH on the oxidation rate of the monoazo dye Acid Orange 7 (AO7), one of the most difficult dyes to degrade, was studied in a homogeneous medium. The results have revealed an increase of nearly two orders of magnitude in the AO7 decolourisation rate when pH increased from 3 to 8.2. This effect was only slightly diminished when the radical scavenger was added. Competitive studies were performed between AO7 and the anthraquinone dye Acid Green 27 (AG27), which exhibits a much faster reaction rate. When the two dyes are mixed together, the AO7 decolourisation rate increases markedly, approaching the AG27 values, even under conditions that favour the undissociated species (pH = 2.7). This is a very positive practical result as the charged ozone can be consumed almost indistinctly by the two dyes. (C) 2012 Elsevier B.V. All rights reserved.