We used GC/MS assisted with elemental analysis to compare the organic compounds in oil, water and gas phases before and after the reaction, and finally deduced the mechanism of the viscosity reduction of heavy oil through catalytic aquathermolysis. The results indicated that the catalyst could not only break the C C bond, but also rupture the C-O bond in the heavy compounds. These changes eventually led to the viscosity reduction. In this work, a new kind of catalyst aromatic sulfonic H3PMo12O40 was firstly synthesized and used to evaluate the viscosity reduction of heavy oil through catalytic aquathermolysis. (C) 2010 Elsevier Ltd. All rights reserved.