The effect of the catalyst load in slurry hydrocracking of heavy Maya crude oil was investigated at mild conditions (390 degrees C and 1400 H-2 psi). The catalyst load and operating time were varied in the intervals 0-1000 ppm Mo and 0-11 h respectively. The results allowed to establish the transformation routes of the different fractions under thermal or catalytic conditions. Under thermal conditions, the contributors to gas formation were found to be: Asphaltenes >> resins > vacuum gas-oil and the use of catalyst inhibited the contributions of resins and vacuum gas-oil. It was found that catalytic slurry hydrocracking evolves in two general reaction stages: the first, dominated by the catalyst-induced reactions, from zero to 50% VR conversion, and the second, above 50% VR conversion, dominated by thermal reactions leading to high production of coke. During the first stage the naturally occurring asphaltenes of Maya crude were effectively transformed to liquid fractions and the formation of coke and new asphaltene-like components were catalytically inhibited. Moreover, the production of middle distillates was enhanced. The second reaction stage in catalytic hydrocracking approaches thermal behavior likely because part of the catalyst is lost when the crude mixture above 50% VR conversion departs from physical equilibrium and forms asphaltenic aggregates that precipitate entrapping some catalyst. During the second reaction stage, middle distillates are transformed to light ends, which are end products because no declination was observed in its production. The use of catalyst improves API gravity and viscosity of the liquid product. In all cases the API gravity increased with naphtha production. The hydrodesulfurization results indicate that most of the eliminated sulfur was of asphaltenic nature, presumably associated to the aliphatic lateral chains of asphaltenes. A qualitative estimation of the importance of the different reaction routes is proposed on the basis of the results of this work. (C) 2014 Elsevier Ltd. All rights reserved.