The sizes and stability of oil droplets created from various concentrations of heavy oil-in-toluene at a fixed oil:water ratio were investigated during turbulent flow in model process water. The Reynolds number (Re) ranged from 17,000 to 34,500 and was obtained by stirred tank mixing with a Rushton turbine. The droplet sizes were monitored using laser light scattering. Results showed that at high Re and low oil concentrations (that is, low drop-surface coverage), breakage of the droplets was the dominant process, but as Re was reduced, coalescence was dominant. Droplets were less prone to breakage as the oil concentrations in toluene increased, and droplet sizes approached a steady state quickly during mixing. Their size distributions broadened and stability increased as heavy oil in toluene increased. Stability was attributed to a surface coverage by asphaltenes and the consequent interfacial elasticity that provided resilience to breakage. Equilibrium interfacial tension sigma(E) was determined by fitting a diffusion-limited kinetic mathematical model to the data. The Gibbs adsorption model gave a monolayer surface coverage of 3 nm(2)/mol asphaltenes, consistent with other published results. High zeta potential of the droplets also hindered coalescence. (c) 2005 American Institute of Chemical Engineers.