As an advanced water treatment, a combination of ozonation and biological activated carbon (BAC, hereafter) treatment is being applied to purify raw water for municipal use in some cases. The authors examined effects of ozonation on water quality in a batch system, using water samples containing organic substances fractionated to several molecular weight ranges. Also, a flow test of laboratory-scale was performed to study on the capability of the treatment in terms of removal efficiency of the dissolved organic substances, e.g., fumic substances, which preoxidized with ozone. As a result, the changes in equilibrium adsorption and in the biodegradability of organic substances dissolved in water before and after oxidation with ozone were made clear.