We have studied the effect of catalytic aquathermolysis of heavy oil in the presence of a hydrogen donor (formic acid) on the viscosity of Liaohe extra-heavy oil. The paraffinic/naphthenic and aromatic hydrocarbon content of the oil as well as the H:C ratio of the oil increase after aquathermolysis in the presence of a hydrogen donor, while the sulfur, resin, and asphaltene content decreases dramatically. We use the thermogravimetric method to show that with aquathermolysis in the presence of formic acid, a substantial portion of the asphaltenes in the heavy oil is converted to paraffins. Accelerating the aquathermolysis reaction results in a synergistic effect between the catalyst and the hydrogen donor.