Ozonation combined with electrolysis (ozone-electrolysis), ozonation alone and electrolysis alone were applied to the treatment of a paper and pulp mill wastewater for the decolorization and removal of chemical oxygen demand (COD). Electrolysis alone was ineffective in treating the paper and pulp mill wastewater. Both ozone-electrolysis and ozonation alone were effective in removing chromaticity, but produced white suspended solids. The production of the solid by ozone-electrolysis was smaller than by ozonation alone. The ozone-electrolysis demonstrated more rapid removal of COD and higher ozone effectivity than the ozonation alone. Furthermore, ozone-electrolysis was the most useful method to transform organic compounds into more oxygenated ones of the three.