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An experimental procedure is described to estimate the extent of evaporative weathering experienced by crude oils spilled in the marine environment. The essence of the procedure is the division of the crude oil into a series of pseudo-fractions by boiling point, and the subsequent modelling of the evaporative behaviour of each with the appropriate n-paraffin. The n-paraffin distributions of a number of naturally weathered crude oils were determined by capillary gas-liquid chromatography using fused silica columns. The actual extents determined by this procedure were compared with those generated by a computer simulation of the weathering, where the evaporative losses were modelled using film thickness, film temperature and weather data. Excellent results were obtained for oil film thicknesses between 10 μm and 1 mm, weathered for periods of up to 4 weeks.