Asphaltenes can cause serious problems to petroleum industry. Indication of asphaltenes stability can be obtained by precipitation induced by n-heptane, monitored by near-infrared (NIR). However, this technique does not provide information on the particles sizes. In this study, the precipitation onset of model systems (1wt% asphaltenes in toluene) was monitored by NIR, with and without adding additive. The particles sizes of the same systems were monitored by atomic force microscopy (AFM): Dispersions of asphaltenes in toluene, n-heptane and mixtures of these two solvents were analyzed by AFM. For the pure model system, the size of the asphaltenes aggregates clearly increased in function of rising n-heptane concentration in the solvent mixture, with this increase being much more pronounced for solubility parameter values below that corresponding to the precipitation onset. In the presence of additives, significantly smaller particles were obtained even when the precipitation onset showed only a slight shift.