The thermal degradation of compounds containing vanadium and nickel in crude oil has been examined in order to throw further light on the significance of the trace metals in the geochemistry of petroleum. Almost identical first-order reaction rates were observed for the removal of the two metals, and extrapolation of the data to geological times and temperatures showed that present-day reservoir temperatures are too low to permit a significant thermal contribution to the changes which appear to have occurred in the maturation of crude oil. A study was made of the degradation of porphyrin compounds which are responsible, in part, for the presence of the metals in oil. The rate for the porphyrin degradation at 358°C. was 0.020 hr.^-1, considerably greater tha that for the removal of vanadium, 0.013 hr.^-1 and the nickel, 0.012 hr.^-1. The activation energy, which is a measure of the temperature dependence of the reaction, was 52.5 Kcal./mole for porphyrin degradation; 58.6 Kcal./mole for vanadium removal; and 57.5 Kcal./mole for nickel removal. These data indicate that the spread between porphyrin degradation rates and metal removal rates would increase at all lower temperatures. Consequently, if thermal action were responsible for the maturation of crude oil in nature, the porphyrin material in a maturing oil would be lost very rapidly compared with the removal of metals. The relatively constant proportion existing between porphyrin content and metal content found for a widely varying suite of Canadian oils shows that th s has, in fact, not happened. This observation tends to confirm the conclusion that thermal action has little to do with the maturation of crude oil.