In this paper, ozonation of organic substances which are found in wastewater is discussed regarding following respects; (1)removal ratios of COD(Cr) and TOC, (2)removal properties, and (3)oxidation products. The main results are summarized as follows. (l)Ozonation quickly removes proteins and amino acids, but mildly polysaccharides and n-saturated carboxylic acids. (2)The initial removal of COD(Cr) or TOC is quite rapid, but, as the reaction continues, it slows down so drasti-cally that one may suspect that it has completed. (3)Starch and albumine are decomposed to the smaller molecular compounds. (4)The COD(Cr) /TOC ratio of each substance which is chosen in this study decreases. This means that ozonation results in the formation of oxygenated fragments of the original molecule-occurs by bond fission or partial oxidation. (5)Proteins are indispensable components in frothing because they are surface active. They are removed by not only oxidation but also foaming, however, the removal by foaming is not so large.