We report a systematic study of the phase memory time (T-M) of normal and deuterated pyrimidine in benzene. We have performed both the Hahn (2-pulse) echo and stimulated (3-pulse) echo experiments in zero magnetic field with optical detection. We focus on the increase of T-M as the guest and the host molecules are successively deuterated. For a guest/host isotopic identity of H/H, D/H, H/D, and D/D, respectively, the 2-pulse echo T-M stands in ratio of 1:1.2:3.0:7.0. This follows the same trend as those in the results of Van't Hof and Schmidt's [Mol. Phys. 38, 309 (1979)] earlier work on quinoline/durene. However, our ratios are much smaller. The corresponding ratio of T-M for 3-pulse echo is 1:2:2:3. After a careful accounting of the contribution of the triplet decay, we have established the existence of a nondecaying residual 3-pulse echo as predicted by Hu and Hartmann [Phys. Rev. 9, 1 (1974)]. We ascribe the smaller deuteration effect on our 2-pulse echo results to the (3)n pi(*) nature of pyrimidine rather than the (3)pi pi(*) nature of quinoline. The small isotope effect on 3-pulse echoes is consistent with a few published reports. A plausible cause for the very slow spin flipping rate extracted from our data is suggested.