Pilot plant studies were carried out on the effect of ozone on the chemical content of a secondary domestic and industrial sewage. Results are expressed in terms of COD, BOD5, nitrite ion concentrations, bacterial counts, XAD-4-extraetable compounds, and free amino acid concentrations. Ozone dosages of 6 to 12 mg/L were found to reduce levels of fecal bacteria, COD and nitrite ion concentrations significantly, to modify the nature and the concentration of XAD-4-extraetable compounds, and to increase the concentration of free amino acids.