Physical interactions of asphaltenes in the heavy oil environment are studied. The approach consisted of following the variation of viscosities of toluene solutions of Heavy Arab vacuum resid containing asphaltenes as a function of: (a) concentration and (b) temperature of the solutions. Based on the results, a mechanism of aggregation consisting of step-wise aggregation followed by cluster formation is proposed. The aggregation is reversible by dilution and by increasing the temperature of the solution.