The organic constituents of wastewaters discharged from the Great Canadian Oil Sands Ltd. (GCOS) plant and the Syncrude Canada Ltd. lease No. 17 were characterized and compared to those occurring naturally in the Athabasca River. Of the 16 chemical groups investigated in samples of upgrading plant effluent, coke storage area runoff, and Syncrude Canada Ltd. mine depressurization waters, the hydrocarbons, organic sulphur compounds, organic nitrogen compounds, and oxygenated organic compounds were found to be the most abundant constituents. Upgrading plant effluents contained an average of 36 mg/l of organic carbon, of which 17 mg/l was extractable with organic solvents. Organic sulphur compounds comprised 24% of the extractable organic carbon, oxygenated compounds 17%, hydrocarbons 16%, asphaltenes 10%, and nitrogen compounds 7%. Coke storage runoff revealed a total organic carbon content of 25 mg/l, 15 mg/l of which was extractable carbon. The extractable portion contained 18% asphaltenes, 13% oxygenated compounds, 11% sulphur compounds, 2% nitrogen compounds, and 1% hydrocarbons. Mine depressurization waters averaged 30 mg/l total organic carbon, 62% of which was extractable. Oxygen-containing compounds were the major contributors to the extractable fraction at 24%, with lesser contributions from sulphur compounds at 15%, asphaltenes 14%, nitrogen compounds 2%, and hydrocarbons 1%. Waters of the Athabasca River upstream from the GCOS plant contained only 13 mg/l total organic carbon, of which 4 mg/l was extractable. The major component of the extractable carbon, 29%, occurred as polar compounds. The hydrocarbon content was low, 0.8% of the extractable carbon, comparable to amounts founds in the mine depressurization waters, but far less than the 16% found in upgrading plant effluents. Under ice conditions on the Athabasca River, the chemical composition of the hydrocarbons 10 km downstream was directly comparable to those found in the upgrading plant effluent, diluted 70-fold. Initial investigations of bottom sediments from the river revealed differences in samples taken upstream and downstream from the oil sands plant. Upstream sediments contained an average of 480 mg/kg of organic carbon and increased 40% in the downstream sediments to 668 mg/kg. Asphaltenes were the major contributor, increasing from 30 to 35% of the carbon content from upstream to downstream samples. Increases also occurred in hydrocarbon content from 7 to 10%; however, polar compounds were relatively uniform in the two locations and averaged 5% of the organic carbon. It was concluded from this and a previous investigation that the main source of light hydrocarbons in the section of the Athabasca River adjacent to GCOS is the upgrading plant, whereas the oxygenated compounds may be derived from the tailings pond dike filter drainage or natural sources, of which mine depressurization waters may be representative. Preliminary evidence indicated that organic materials released by the plant are fractionated in the river system with the sediments acting as a sink, particularly for heavier hydrocarbon components.