The ozonation of 2-nitro-p-cresol and p-toluenesulfonic acid (c = 1 mmole/l, ozone dose 10 mg O3/min l) in aqueous solution as a function of the pH-value (pH = 2,5 - 10,5) was investigated. At pH = 2,5 the reaction products of 2-nitro-p-cresol are measured quantitatively over the run of the reaction. The results show that the elimination rates for the initial compounds and the mineralization of the heterogroups increase with the pH value. On account of this result the spectrum of oxidation products is different after 90 % elimination of the initial compound as a function of pH. The values of DOC-and COD-elimination increase with increasing pH-value. Although the ozone absorption is better in the basic range one finds that 1,6 - 2,2 mg O3/mg ∆ COD are required in the basic range (pH 10), but only 1,1 mg O3/mg ∆3 COD in the acid range (pH 3).