A fugacity-based methodology is presented to predict the fate of spilled oil in the marine environment. In the proposed methodology, oil weathering processes are coupled with a level IV (dynamic) fugacity-based model. A two-compartment system, comprised of water and sediment, is used to explore the fate of oil in a marine environment. During a spill, oil is entrained into the water column due to natural dispersion, which is considered as the primary input source to the water compartment. The direct input to the sediment compartment is assumed negligible. However, the water column acts as a source to the sediment compartment. Unlike the conventional multimedia modelling approach, the impact area is not predefined. Instead, the oil slick spreading process determines the contaminated area growth. Naphthalene is used as a representative oil compound (an indicator) to demonstrate the application of the methodology. The current study suggests that the water compartment response to the chemical input is faster than the sediment compartment. The major fate processes identified are advection in water and volume growth in the sediment. (C) 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.