Asphaltenes of Maya and Isthmus crude oils were precipitated, fractionated and characterized in this work. Isolation of asphaltenes was performed by following the ASTM D3279 method, which uses n-heptane for solvent precipitation. Asphaltenes were separated into three fractions by Soxhlet extraction with a binary solvent system of toluene and n-heptane. C, H, O, N, S, and Ni and V contents were determined in asphaltenes and in their fractions by elemental analysis and atomic absorption, respectively. VPO aggregate weight and NMR measurements were also performed in all samples. Important differences in properties of unfractionated asphaltenes and asphaltenes fractions were observed. Some of these differences were attributed to impurities in the unfractionated asphaltenes. (C) 2004 Elsevier Ltd. All rights reserved.