Determination of ozonation by–products using five organic compounds and secondary effluents was carried out. Formaldehyde, acetaldehyde, propionaldehyde, furfural, methlyglyoxal, glyoxylic acid, and pyruvic acid were detected by gas chromatography of their 2,4–dinitrophenyl–hydrazones. Acetic acid, formic acid, and propionic acid were also detected. One–sixth to one–fifth of the total COD was accounted for by the TOD summations of formaldehyde, methylglyoxal, pyruvic acid and acetic acid produced by the ozonation of secondary effluents.