In the crystal structure of 6-phenyl-3-thioxo-2, 3, 4, 5-tetrahydro-1, 2, 4-triazin-5-one, C9H7N3OS,(I), the 1, 2, 4-triazine moieties are connected by face-to-face contacts through two kinds of double hydrogen bonds (N—H⋯ O and N—H⋯ S), which form planar ribbons along the a axis. The ribbons are crosslinked through C—H⋯ π interactions between the phenyl rings. The molecular structures of two regioisomeric compounds, namely 6-phenyl- ...