A study of the influence of reaction parameters (reaction temperature, pressure and catalytic precursor) on the upgrading of Maya heavy oil under low pressure and moderate temperature under slurry conditions is presented. The behavior of the purely thermal hydrotreating system was first characterized to have a basis for distinguishing the effect of incorporating a catalyst in further hydrotreatings. The distillation curves of liquid product were obtained by thermo gravimetric analysis (TGA), ASTM standards were used for evaluation of API gravity and content of asphaltenes and carbenes-carboids. The separated asphaltenes were analyzed by XRD and C-13 NMR techniques. The effect of temperature, catalytic precursor and pressure (800 and 960 psi) of process were studied. The study of thermal hydrotreating showed that 390 degrees C and 400 degrees C were promising temperatures for catalytic hydroprocessing and increasing pressure approximately 100 psi promoted the improvement of light liquid fractions at the expense of the heavy and residua fractions. The use of catalytic precursors at 400 degrees C in concentrations of 330 ppm of Mo increased gasoline production without producing more solids but did not result in the improvement of the liquid yield in comparison to the homologous thermal hydrotreatment. The liquid yield was improved either hydrotreating at 390 degrees C or at 400 degrees C with 1000 ppm Mo as catalyst. (C) 2012 Elsevier Ltd. All rights reserved.