Combustion-tube experiments clearly showed the cause and effect of low-temperature oxidation (LTO) and high temperature oxidation (HTO) for heavy crude oils. LTO occurred due to the low fuel concentration, which resulted in (i) a reaction-front temperature of only 350 degrees C, compared to 500 degrees C for HTO, and (ii) oxygen moving ahead of the reaction front and oxidizing the crude oil. Consequently, an oxygenated hydrocarbon fuel was formed, while the produced oil viscosity and specific gravity increased. A method was derived to estimate the heats af reaction for oxygenated hydrocarbon fuels. During HTO, practically all of the oxygen injected was consumed, so that the fuel was not oxygenated. As a result of distillation, viscosity and specific gravity of the produced oil during HTO decreased significantly.