A heavy crude oil and its upgraded oils, obtained by batch slurry-phase hydrocracking at low-severity conditions with and without analytical-grade and ore catalysts rich in iron and molybdenum, were fractionated by atmospheric distillation and were deasphalted with the aim of identifying the fraction responsible for the upgrade of the flow properties. Light cuts were separated from the distillation bottoms at 260 degrees C. This latter fraction was further deasphalted with heptane at 25 kg/cm(2) and 60 degrees C for separation of maltenes and heptane insolubles. The heptane insolubles were subsequently fractionated into asphaltenes and toluene insolubles by Soxhlet extraction. Light cuts, bottoms, and maltenes were characterized by density, viscosity, simulated distillation, and elemental analysis. Light cuts were also characterized by PIONA analysis. Elemental analysis, H-1 and C-13 nuclear magnetic resonance, X-ray diffraction, and molecular-weight distribution by gel permeation chromatography were carried out for asphaltenes and toluene insolubles. The upgrade of the flow properties of hydrocracked products was found to be due to the increase of the light cut and maltene contents, which was caused by the hydrocracking of asphaltenes as well as the upgraded properties of the heavy crude oil, which are proportional to the active metal content and the hydrogenation capacity of the catalysts (Mo > Fe).