The mol­ecules of 2-[6-amino-3-methyl-2-(methyl­sulfanyl)-4-oxo-3,4-dihydro­pyrimidin-5-yl­carbonyl]­acetonitrile, C9H10N4O2S, (I), are linked in pairs by N—H⋯O hydrogen bonds to form cyclic centrosymmetric R22(4) dimers. Similar dimers formed by 2-(6-amino-1,3-dimethyl-2,4-dioxo-1,2,3,4-tetra­hy­dro­pyrimidin-5-yl­carbonyl)­acetonitrile, C9H10N4O3, (II), are reinforced by paired N—H⋯N hydrogen bonds and linked into chains of rings by C—H⋯O hydrogen bonds. The mol­ecules of 2-cyano-N-[6-meth­oxy-2-(methyl­sulfanyl)­pyrimidin-4-yl]­acetamide, C9H10N4O2S, (III), are linked into simple C(6) chains by an N—H⋯N hydrogen bond, and the chains are weakly linked into sheets by a π–π stacking inter­action. A combination of one two-centre N—H⋯N hydrogen bond and one three-centre C—H⋯(N,O) hydrogen bond links the mol­ecules of 2-cyano-N-[6-chloro-2-(methyl­sulfanyl)­pyrimidin-4-yl]­acetamide, C8H7ClN4OS, (IV), into a chain of alternating edge-fused R21(6) and R12(6) rings. The crystal structures reported in this study, and those of some related examples from the recent literature, show a wide variation in hydrogen-bonded aggregation consequent upon rather small changes in molecular constitution.