A MoO3/deep eutectic solvent (DES) catalyst precursor solution was prepared by dissolving molybdenum trioxide in a DES based on choline chloride/urea. The catalyst precursor solution was characterized for its physical and chemical properties. The characterization results showed almost no change in the DES properties after the addition of MoO3. The solution was used in the catalytic upgrading reaction of heavy crude oil. The performance of the catalyst was analyzed by gas chromatographymass spectrometry, Fourier transform infrared, and viscosity measurements of the heavy oil before and after the reaction. The use of the catalyst in the catalytic aquathermolysis showed an increase in the oil viscosity. In the presence of hydrogen and catalyst, the results showed a 43% reduction in the heavy oil viscosity, a 2.5 degrees increase in the API gravity, and 32 wt % sulfur reduction.