The paper mill industry produces high effluent volumes that normally cause a high environmental impact. In this work, Kraft E-1 effluent was treated with ozone and some chemical parameters (total organic carbon, molecular mass distribution and total phenols) were analysed at different ozonization times. The toxicity of the effluent was determinated during the ozonization process, using the bacterium Escherichia coli, by a respirometric test. After sixty minutes of ozonization, total organic carbon decreased to around 12%, total phenols to 70% and effluent colour to 35%. The effluent molecular mass was significantly reduced during the process and the toxicity to E. coli decreased with effluent ozonization.216717721