4-Amino-1-benzyl-2-(methylsulfanyl)pyrimidin-6(1H)-one, C(12)H(13)N(3)OS, crystallizes in two polymorphic forms, both having space group P2(1)/c, with Z' = 1 for form (I) and Z' = 2 for form (II). In (I), the molecules are linked by a single N-H.O hydrogen bond into simple C(6) chains, which are themselves linked into sheets by aromatic pi-pi interactions, while in (II), chains of edge-fused R(4)(2)(8) and R(4)(4)(24) rings are generated by four distinct N-H.O hydrogen bonds. In 4-amino-1-benzyl-2-(methylsulfanyl)-5-nitrosopyrimidin-6(1H)-one, C(12)H(12)N(4)O(2)S, (III), where Z' = 2, two independent three-centre N-H.(N,O) hydrogen bonds generate a C(5)C(6)[R(1)(2)(5)] chain of rings. In 4-amino-6-benzyloxy-2-(methylsulfanyl)pyrimidine, C(12)H(13)N(3)OS, (IV), which is isomeric with (I) and (II), a combination of N-H.N and N-H.O hydrogen bonds generates a sheet of alternating R(2)(2)(8) and R(6)(6)(28) rings.