When spilled in the environment, especially in coastal systems such as estuaries, oil frequently interacts with fine mineral particles to form aggregates. This phenomenon may be enhanced in the case of chemical dispersion and influence the behavior and fate of the pollutant in the environment. Understanding this process will help decide whether chemical dispersion is a good oil clean-up option in a particular environment. This study investigated the formation of oil-mineral aggregates (OMA) when the oil was chemically dispersed, focussing on the size distribution of these structures. Results of laboratory experiments show that aggregate size is correlated to its relative composition in oil and clay, and that for a given concentration of mineral, the average size presents a maximum. Other highlights include the influence of oil type and salinity on the clay concentration corresponding to maximum size. The behavior of a particular oil as OMA depends on the size and buoyancy of its aggregates which will vary with the local of salinity, suspended mineral load and hydrodynamics conditions. (C) 2003 Elsevier Science Ltd. All rights reserved.