Py-GC/MS analysis of bottom sediment from Lake Ontario was shown to be effective in assessing organic contaminants, especially attractive as it is a rapid technique requiring little sample preparation. Pyrolyzates of samples richer in organic matter (OM) were relatively enriched in aliphatic hydrocarbons and pyrrolic nitrogen compounds, while leaner samples were more aromatic and pyridinic. Alkylbenzene and alkylphenol distributions in the pyrolyzates were most compatible with derivation from aquatic (algal, bacterial) OM. The organonitrogen compounds indicated the presence of degraded proteinaceous material from aquatic sources and/or sewage. Normal and polycyclic aromatic hydrocarbon distributions indicate, at least in part, contributions of fossil fuels and combustion residues. These observations were confirmed by organic petrologic examination. The ubiquitous input of aquatic OM to the sediments (enhanced by anthropogenic nutrient oversupply) was supplemented by fossil fuel-derived contamination, in proportions varying with proximity to industrial sites and shipping lanes.