2-Amino-4-chloro-5-formyl-6-[methyl(2-methylphenyl)amino]pyrimidine, C(13)H(13)ClN(4)O, (I), and 2-amino-4-chloro-5-formyl-6-[(2-methoxyphenyl)methylamino]pyrimidine, C(13)H(13)ClN(4)O(2), (II), are isostructural and essentially isomorphous. Although the pyrimidine rings in each compound are planar, the ring-substituent atoms show significant displacements from this plane, and the bond distances provide evidence for polarization of the electronic structures. In each compound, a combination of N-H...N and N-H...O hydrogen bonds links the molecules into sheets built from centrosymmetric R(2)(2)(8) and R(6)(6)(32) rings. The significance of this study lies in its observation of the isostructural nature of (I) and (II), and in the comparison of their crystal and molecular structures with those of analogous compounds.