A sample of asphaltenes (ACN) precipitated from Cerro Negro crude oil had been divided into 14 fractions using a dialysis procedure employing THF-acetone mixtures as the extracting solvent. Seven extracts (F-1-F-7) and the corresponding residues R-1-R-7 were obtained as the mixture composition changed from 40 to 100% THF. These materials were characterized by elemental analyses, VPO, solubility, C-13 NMR, and electron paramagnetic resonance (EPR). As the last fractions are aproached, the following trends were observed : WC decreases, aromaticity increases, solubility descreases, and S-d, the spin density, increases. Removal of the first fraction (F-1, 15% of ACN, acetone-THF (40%)) afforded a residue (R-1) insoluble in toluene, indicating that 85% of ACN is present as colloidal particles (R-1) dispersed in this solvent by F-1. More than 12% of ACN found in the last residues (R-6 and R-7) were found to be insoluble in organic solvents, suggesting that these fractions are formed by aggregates of molecules held in place by strong intermolecular forces. in an asphaltene micelle they would be at the core and its solubility in organic solvents is due to dispersion by the other components of the micelle. Althought other effects are not disregarded, it is suggested that transannular electron delocalization in free radicals could play an important role in aggregate formation.