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Asphaltenes, defined as the fraction insoluble in n-heptane, are responsible for flocculation of crude oil. We consider both crude oil and asphaltene solutions as dynamical colloidal systems containing particles that may form different aggregates upon changing the temperature and polarity of the solvent. For example, resins, the polar molecules present in crude oils, constitute a good solvent for asphaltenes. We used ultra small-angle x-ray scattering (USAXS) in order to investigate the evolution of the macrostructure of asphaltenes, resins, and crude oils. This technique enables to investigate particles of the size range 10-10000 Å. We have studied a simplified system composed of asphaltenes and resins in a toluene solution, as well as the vacuum residue that contains asphaltenes, resins, aromatics, and saturated species. An important finding is the presence of large-size fluctuations of concentration in the temperature region 200°∮300°C. The stability of agglomerates is caused by covalent bonding. However, we observe the decrease of aggregate size for asphaltenes and resins at higher temperatures. The rheology results confirm flat shape of the asphaltene's aggregates. The effect of temperature is particularly pronounced for small aggregates of asphaltene molecules. [ABSTRACT FROM AUTHOR]