The electric quadrupole coupling constants eqQ/h of 20 Na (I p = 2 + , T 1/2 = 449.7 ms), 21 Na (I p = 3/2, T 1/2 = 22.49 s) and 25 Na (I p = 5/2, T 1/2 = 59.6 s) in single crystal ZnO and TiO 2 were precisely measured by applying the -NMR technique. The ratio of the quadrupole moments between 20 Na ( 25 Na) and 21 Na was determined to be 0.728 T 0.023 (0.011 T 0.002).