C-13 nuclear magnetic resonance chemical shifts of 1-substituted-3, 4-dimethoxy-and-3, 4-methylenedioxybenzene derivatives were measured. Reliability of the additivity rule of the shielding parameters of monosubstituted benzenes, and the linear relations between the chemical shifts and shielding parameters or substituent constants were confirmed. Deviations from the additivity rule are mainly attributed to the ortho steric effect.