Treatment with ozone and ozone/hydrogen peroxide was tested in a laboratory scale reactor for removal of organics from four different industrial wastewaters: wastewaters of a paper-mill and of a biotechnical pharmaceutical process as well as two process waters from soil remediation by supercritical water extraction. Moreover, an aqueous solution of triethyleneglycoldimethylether and humic acid which was a model for a biologically treated oil reclaiming wastewater was also oxidized. The aim of the oxidation of the pharmaceutical wastewater was the removal of the preservative 1.1.1-trichloro-2-methyl-2-propanol (TCMP). Although TCMP could easily be removed from pure aqueous solutions by treatment with ozone/hydrogen peroxide, the oxidation of the wastewater failed to be effective in TCMP degradation because of competitive ozonation of other organic solutes in the wastewater. The ozonation of the paper-mill wastewater and of the soil remediation process waters decreased COD and TOC to some extent. The presence of organic wastewater solutes which contain C-C double bonds (ligninsulfonic acid in the treated paper-mill effluent and humic acid in the oil reclaiming model wastewater) were shown to yield hydrogen peroxide by the reaction with ozone. Therefore, these wastewaters are efficiently ozonated even without addition of hydrogen peroxide. Chemical Oxidation of paper-mill wastewater and of wastewaters resulting from soil remediation did not improve biological degradability of organic wastewater constituents.