A photometer was utilized to measure the intensity of scattered light in toluene / heptane / asphaltenes mixtures. On the scattering-intensity versus heptane-to-toluene ratio functions, two linear regions can be observed. The intersection of these regions determines the heptane-to-toluene ratio at which the precipitation of asphaltenes starts. The onset of asphaltene precipitation was determined as a function of asphaltene concentration in the presence and in the absence of maltenes as well as water. The onset is dependent on the asphaltene concentration. The presence of water promoted while the presence of maltenes inhibited the precipitation of asphaltenes.