The effect of ozonation process on pulp and paper mill effluents was investigated. The objectives were to: (1) identify various compounds in wastewater from a pulp and paper mill, (2) evaluate decolorization and organic removal efficiency by conventional bubble reactor and (3) evaluate the biodegradability at various progressive stages of ozonation. The qualitative GC/MS analyses were performed before and after the biological treatment and ozonation process. Two groups of compounds were observed in this wastewater: lignin-derived compounds and aliphatic compounds used in the pulp and paper production process (i.e. n-alkanes, fatty alcohols, fatty acid and ester). Treatment efficiency was measured by decolorization and TOC removal rates. Additionally, the utilization coefficient (k) and BOD/COD ratio were determined to observe the biodegradability of ozonized effluents. The results indicated that after 45 min, the ozonation of effluents yielded almost colorless effluent with over 90% decolorization efficiency and with corresponding ozone capacity rate of 20.0 mg O₃ L⁻¹. This decolorization was not always accompanied by the mineralization of the organic matters therefore ozonation was not related to TOC removal rates. The BOD/COD ratio increased from 0.10 to a maximum value of 0.32 with ozone flow rate (O/F) of 4.0 L min⁻¹. It was confirmed by the utilization coefficient as first order BOD equation, the magnitude k value increased from 0.21 day⁻¹ to maximum value of 0.47 day⁻¹ as the ozonation time was raised to 60 min with O/F 4.0 L min⁻¹.