A second monoclinic polymorph of 6-amino-1,3-dimethyl-5-[(E)-2-(methyl-sulfan-yl)benzyl-idene-amino]-pyrimidine-2,4(1H,3H)-dione.

A new monoclinic form of the title compound, C(14)H(16)N(4)O(2)S, has been identified unexpectedly during an attempt to synthesize a coordination compound. The heterocyclic ring is essentially planar (r.m.s. deviation = 0.005 Å) and makes a dihedral angle of 8.77 (5)° with the benzene ring. This is in contrast to 12.24 (7)° reported for the first monoclinic polymorph [Booysen et al. (2011 ▶). Acta Cryst. E67, o1592]. An intra-molecular N-H⋯S hydrogen bond is observed. In the crystal, inter-molecular N-H⋯O hydrogen bonds link the mol-ecules into zigzag chains along the b axis. The closest distance between the centroids of symmetry-related heterocyclic rings is 3.5161 (6) Å.