of 1-methylcyclohexyl acetate-2,2,6,6-d4, the deuterium isotope effect for the acetate pyrolysis reaction has been determined to be 1·7 at 500° and 1·9 at 400°. These values are to be compared with the theoretical maximum values of 2·1 and 2·3 respectively. By carrying out competitive pyrolyses it has been possible to determine some relative rates of pyrolysis which show that no appreciable positive