The conversion of asphaltenes in heavy oil hydroprocessing is important as the asphaltenic fraction often contains the most refractive components. In residual oil feeds, the content of asphaltenic sulfur amounts to less than 20% of the total sulfur content; in hydrotreated oil products, however, more than 60% of the sulfur content may be asphaltenic. When hydrotreating heavy oils, both the non-asphaltenic phase and the asphaltenes undergo a chemical conversion, which may lead to unstable products as well as to increased coke lay-down on the catalyst. To be able to explain the behavior of different catalysts, the asphaltene conversion and particularly the stability of asphaltenes in hydrotreated heavy oil products were investigated. The stability of the asphaltenes was characterized by a flocculation onset titration procedure. With this method, several different concentrations of oil in toluene were titrated with heptane, and the stability parameters were derived. This study shows how the product stability is affected by changes in reaction temperature. The critical solubility parameter for the solubility of asphaltenes follows the assumed chemistry of the hydrotreating processes in question. At low severity hydroprocessing, the stability of the product increases, but when the severity is increased, the asphaltene stability is reduced.