By measurements of static light scattering, optical absorption, dynamic viscosity and NMR relaxation we have studied formation of asphaltene dispersions in oil/toluene solutions with asphaltene concentrations from 1.6 to 400 mg/L. The observed threshold concentration for self-assembly of asphaltene monomers is below 10 mg/L, almost three orders of magnitude lower than the frequently reported 'critical concentrations' of asphaltene aggregation. The specific behaviour of the measured parameters at concentrations close to 100-150 mg/L we attribute to a phase separation. The discussion of the results suggests the existence of a solvophobic asphaltene subfraction singled out by its extremely low solubility. Hence, aggregation of asphaltenes appears to be controlled by solvent-mediated attraction, determined by the solvophobic effect. 'Insoluble' asphaltenes are thought to form the stacked cores of colloidal particles while the more soluble asphaltene fraction may provide surrounding stabilizing shells/coronas. (C) 2006 Published by Elsevier Ltd.