Wet oxidation (WO) of diluted aqueous azo dye solutions was carried out with and without a catalyst. The results showed that the presence of a solid catalyst yields much smaller amounts of intermediates and that the maximum of their occurrence is shifted towards shorter residence times. Although the intermediates formed during oxidations are identical, in addition to the different intermediate product distribution their lower amounts are the most responsible for the higher biodegradability found with effluents treated catalytically.