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Much interest has recently focused on the quantitative apportionment of multiple sources of toxic and carcinogenic compounds in natural aquatic systems. Apportionment studies of polycyclic aromatic hydrocarbons (PAH) are of great interest because, in addition to their suspected toxic and carcinogenic properties, they also have a wide range of potential natural and anthropogenic sources. This paper presents the results of a study where the primary source inputs of PAH to sediments of St. John's Harbour are quantitatively assessed using a combination of molecular abundance and carbon isotope measurements of individual (4-and 5-ring) PAH. Mass balance calculations using a two-component mixing model show that approximately 50-80% of the PAH input to the Harbour sediments is of combustion origin, likely dominated by vehicular emissions carried by surface runoff from the city of St. John's. Direct petroleumrelated contribution, possibly dominated by crankcase oil, accounts for the remaining 20-50% of the total PAH input.