The catalytic effects of minerals on aquathermolysis of heavy oil were studied. The change of viscosity and average molecular weight of heavy oil with temperature, water content and reaction time are discussed in the paper. The results show that heavy oil can undergo aquathermolysis in steam injection condition. The results also show that minerals can accelerate the aquathermolysis of heavy oil and lead the viscosity and average molecular weight to decrease further. The reservoir minerals have the catalytic effects on the aquathermolysis of the heavy oils. The results provide a basic theoretical basis for down-hole catalytic upgrading of heavy oil during steam stimulation for heavy oil recovery. (C) 2004 Elsevier Ltd. All rights reserved.