Iron oxide (magnetite) particles are synthesized. Their effects on the transformation of high-molecular-mass components of Ashal'cha deposit heavy oil after simulating steam-heat action are studied. The contents of resinous asphaltenes are found to decrease if a catalyst is present in the oil. The viscosity decreases during hydrothermal catalytic action. Starting catalyst of mixed Fe(II, III) oxides and catalyst recovered from the oil were analyzed using x-ray structure analyses. Catalyst particle sizes were established using scanning electron microscopy.