The chemical oxygen demand (COD) change of various substituted aromatic compounds, namely benzoic acid, p-aminobenzoic acid, p-toluenesulfonic acid, sulfanilic acid: nitro benzene, resorcinol, p-cresol, o- cresol, o-toluidine, aniline and 8-hydroxyquinoline were investigated at ozone doses of 5, 10, 15 and 20 mg O-3. min(-1), respectively. Percent COD removal of initial compounds after ozonation was compared with reported biodegradation results. The pH change and percent removal of selected compounds were also evaluated in ozone does of 20 mg O-3. min(-1) up to 80 min. ozonation. The results showed that high ozone doses are needed to obtain better elimination of initial compounds and also to improve further degradability of the ozonation products.