Abstract: Hydrogen isotope effect is found in the catalytic addition reaction-synthesis of (4,5-3H)-exo-cis-3, 6-epoxyhexahydrophthalic anhydride and the isotope separation factor is related to the temperature by the equation lg α- 0.04 + 138/T. Theoretical values of specific activities of the 3H-1abelled compound calculated from the equation agree well with experimental values. The agreement proves that hydrogen isotope effect does exist on the palladium catalyst.