A study of the physical, chemical, and toxicological properties of untreated and treated tar sand wastewaters has been completed. One of the waters selected for this study, TS-1S wastewater, was generated during an in situ steam flood experiment conducted by the US Department of Energy. The other waters were generated during laboratory-scale extraction experiments that were designed to test high-pressure steam displacement and reverse combustion as processes for recovering oil from tar sand. One objective of this study was to evaluate the effectiveness of four treatment processes for reducing the contaminant load and toxicity of TS-1S wastewater. The treatment processes included: (1) ferric chloride coagulation, (2) ferric chloride coagulation plus activated carbon adsorption, (3) polymer-assisted flotation, and (4) polymer-assisted flotation plus activated carbon adsorption. According to the results of chemical analyses, the effluents generated by these treatment processes were very similar. However, polymer-assisted flotation plus activated carbon adsorption was most effective for reducing toxicity, whereas ferric chloride coagulation appeared to increase toxicity to at least one of the organisms tested. Thus, standard physicochemical analyses were not adequate indicators of the potential environmental hazards of TS-1S wastewater, as determined by toxicity testing. This study indicated that physical, chemical, and toxicological characterization is necessary to fully assess the potential environmental hazards of tar sand waters or any other chemically complex water. Another objective of this study was to expand the data base for tar sand wastewaters. 18 refs., 8 figs., 10 tabs.