The environmental fate of Iranian Heavy crude oil (IHC) with and without an added oil spill dispersant (OSD) has been studied using a 1000 kL capacity in situ mesocosm. Physical weathering and chemical composition changes of the oil were monitored for 77 days. Compound-specific effects of the OSD could be observed as changes over time in the content of the total petroleum hydrocarbon (TPH), unresolved complex mixture (UCM), alkanes, polycyclic aromatic hydrocarbons (PAHs), hopanes and steranes in the oil. As oil weathers, most hydrocarbons showed a rapid decreasing phase followed by a slowdown and stabilization. Recalcitrant biomarkers, however, showed a different trend. An increase in hydrocarbon contents in the form of UCM occurred after OSD treatment. The enhanced solubility of the low molecular weight PAHs by the OSD decreased the half-life of the alkylated PAHs in the OD. After 77 days of exposure at the sea surface, both the oils with and without the OSD exhibited moderate weathering. Most of the source diagnostic indices maintained their source information, and the weathering indices indicated that evaporation, dissolution, and dispersion were the major weathering processes. The mass balance of the weathered oil was calculated using laboratory and mesocosm data and the results demonstrate the importance of using a mesocosm for the production of environmentally realistic data. (C) 2013 Elsevier B.V. All rights reserved.