The structural and dispersion characteristics of samples of heavy crude oils and fractions isolated from them have been studied. A method of in situ analysis of the aggregation of asphaltenes directly in crude oils on the basis of smallangle X-ray scattering is proposed. It has been shown that the maximum size of scattering particles in all crude oil samples is limited to the value on the order of 8.0 nm; the average diameter is ˜2.0 nm. It has been found that the fraction with sizes of 0.8-2.5 nm is mostly composed of resins. Asphaltenes in crude oils form larger aggregated particles with a size up to 8 nm. A comparative study of model dilute solutions of asphaltenes in toluene (0.23 wt %) has shown that the major part of asphaltenes in the liquid (˜94%) is in the form of individual molecules with a size of 0.4-1.2 nm, and only an insignificant remaining part (˜6%) occurs in the form of large aggregates