Interparticle interactions of the soluble asphaltenes in partially deasphalted heavy oils in toluene-heptane (heptol) mixtures are compared to those of several heavy oils diluted in toluene only. Viscosity-volume fraction (eta-Phi) relationships for the heavy oils and bitumen in toluene were almost identical. However, the asphaltenes in toluene associated and scaled differently from its source oil. Four classical viscosity models were used to describe the data, and scaling was interpreted on the basis of asphaltenes association, as in macromolecular interactions, The Pal-Rhodes model showed deviation from sphericity with solvation constants for heavy oils in toluene and C-5 asphaltenes in toluene, at 1.4-1.6 and 3.7, respectively. The Krieger-Dougherty (KH) model indicated high interparticle interaction factors, and maximum packing factors of similar to 1 suggested polydispersity. Neither models fit the data for deasphalted oils. The Leighton-Acrivos model showed that W the maximum packing fraction (Phi(max)) for all oils was similar, (ii) the asphaltenes alone in toluene had the highest self-associations, and (iii) the deasphalted oils showed Phi(max) values close to the theoretical values (0.58). From the Einstein equations, intrinsic viscosities [eta] of deasphalted oils in heptol gave aspect ratios (length to radius, L/R) of the asphaltenes at 10 (i.e., rodlike molecules). The K-H model gave [eta] of similar to 4 and L/R approximate to 3.5 for heavy oils in toluene; however, for asphaltenes in toluene, the model gave [eta] approximate to 10.6 and L/R approximate to 5.8 (i.e., less-rodlike molecules).