Abstract <p>This paper reports on the determination of a stability threshold for an 0.1 g/L asphaltene solution in toluene with the addition of heptane as a precipitant. For this purpose, ultramicroscopy as well as dynamic and static light scattering methods were employed. Ultramicroscopy has never previously been used to determine stability thresholds in petroleum systems. Using the abovementioned experimental methods, an asphaltene aggregation onset point was identified for the tested asphaltene solution, and a time-dependent trend in mean aggregate size was revealed once the stability threshold was exceeded. The study results show that ultramicroscopy enables rapid onset point determination in model petroleum systems of this type and provides higher sensitivity than dynamic or static light scattering; specifically, it can detect lower concentrations of asphaltene aggregates. A single, bulk addition of heptane above the onset point resulted in diffusion-limited aggregation. In contrast, gradual titration past the stability threshold led to a slower process in which the mean aggregate size increased linearly with time.</p>