Hydrotreating of Maya crude was carried out in a pilot plant at different pressure, temperature and space-velocity, keeping constant the hydrogen-to-oil ratio. Pressure was varied in the range of 70-100 kg/cm(2), temperature between 380 and 420 degrees C, and space-velocity between 0.33 and 1.5 h(-1). Asphaltenes were precipitated from the feed and from hydrotreated products by using a batch reactor with n-heptane as solvent. Asphaltenes were characterized by elemental analysis, metals content, VPO aggregate molecular weight and NMR measurements. The effects of reaction conditions on asphaltene properties during hydrotreating are discussed in terms of changes in heteroatom contents and structural parameters. (c) 2005 Elsevier B.V. All rights reserved.