Fractionation of petroleum asphaltene in toluene has been carried out using ultracentrifugation. The separated asphaltenes are examined by small-angle X-ray scattering (SAXS). The radii of gyration and molar mass of the separated asphaltenes range over a wide scale that provides a good indication of the self-similar structure of asphaltene aggregates. The relations between mass and radius of gyration (M proportional to R-G(D)) and between scattered intensity and wave vector (l(q) proportional to q(-d)) were studied. Values of both D (= 1.99) and d (= 2.06) show a fractal dimension D-f of the asphaltene structure close to 2. The mass distribution of these aggregates can be calculated from the knowledge of both the quantity of separated fractions and the mass M of every fraction. We found experimentally a mass distribution with a power-law shape (N(M) proportional to M-tau) with exponent tau = 1.66. The fractal dimension values and the mass distributions of asphaltene aggregates are similar to those predicted by the RLCA model. (C) 2000 Elsevier Science Ltd. All rights reserved.