Extensive small-angle X-ray and neutron-scattering data, as well as results of precipitation measurements, are analyzed to delineate the structure of the asphalt and asphaltene aggregates that are formed when a solvent is injected into a system containing crude oil. The two types of data strongly suggest that both small and large aggregates have a fractal structure, with well-defined fractal dimensions. If the system is aged long enough at low enough temperature, large asphalt particles will have the structure of diffusion-limited cluster-cluster aggregates with a fractal dimension D-f similar or equal to 1.8, while the small ones are similar to diffusion-limited particle aggregates with a fractal dimension d(f) similar or equal to 2.5. High temperatures increase the rotational motion of the particles, disturb the structure and mechanical stability of the aggregates, and decrease their fractal dimension. Aging and concentration effects of the asphalts in the solution, and the type of the solvent on the structure of the aggregates are also investigated. Implications of these results for the structure, mechanical stability; and molecular-weight distribution of asphalts and asphaltenes are detailed. A new molecular-weight distribution for asphalt aggregates predicts the experimental data excellently.