Asphaltenes are broadly defined as the nonvolatile fraction of a crude oil that is insoluble in n-heptane. 1 This fraction contains mixed paraffin—naphthene—aromatics inpolycyclic structures plus sulfur, oxygen, and nitrogen derivatives. Yen et al. 2-4 proposed a structural model of solid asphaltene particles made of stacks of a few aromatic sheets surrounded by some aliphatic chains. In the liquid phase, there is probably a colloidal polydispersion in both size and shape. In recent years, the colloidal structure of asphaltene solutions was a subject of intensive study by X-ray and neutron scattering, 5-7 dielectric spectroscopy, 8 and luminescence analysis. 9 However, the processes near the onset of incipient coagulation and the precipitation kinetics havenot been studied so well. 10 These processes are of great importance for many applications such as deasphaltization, catalysis, bitumen technologies, etc. In this paper we present the results of a study by photon correlation spectroscopy of the coagulation kinetics of asphaltene aggregates in mixtures of a solvent (toluene) and a precipitant (n-heptane). Of course, the process of asphaltene flocculation depends on many factors (eg, the nature of the solvent/precipitant system). It is clear that the flocculation processes in crude oils can be strongly different from those observed in our model experiment. 21 However, to understand the basic rules of the flocculation behavior of asphaltenes, it is convenient to use the simplest model system toluene/heptane.