Ultracentrifugation has been used to produce asphaltene fractions of reduced polydispersity. The structure of these asphaltene fraction solutions has been investigated using viscosity and X-ray scattering (SAXS) measurements as a function of concentration. The relative viscosities of the solutions were found to be fraction-dependent: intrinsic viscosities, radii of gyration, and second viriel coefficients followed a power law with molar mass M-w. A flat disc model succeeded in describing scattering data but failed to take viscosity data into account. By contrast, a fractal model has been found to be consistent with dependence of all measured parameters. Asphaltene-in-toluene solutions were found to form nanometric mass fractal aggregates of fractal dimension 2.1, which in consequence trapped solvent. When, instead of concentration, effective volume fractions are used, the relative viscosities of fractions merge on a master Curve which can be fitted by a hard sphere model. In addition, the reduced osmotic moduli deduced from scattering measurements of the different solutions, when expressed as a function of a concentration adimensional parameter, merge again on a master curve which is in accordance with the hard sphere behavior. The viscosities of solutions can be fully predicted from structure considerations if the ratio of hydrodynamic to gyration radius is taken as 0.6. This ratio is found consistent with the fractal description of the aggregates.