In the crystal structure of 6-phenyl-3-thioxo-2,3,4,5-tetrahydro-1,2,4-triazin-5-one, C(9)H(7)N(3)OS, (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...pi interactions between the phenyl rings. The molecular structures of two regioisomeric compounds, namely 6-phenyl-2,3-dihydro-7H-1,3-thiazolo[3,2-b][1,2,4]triazin-7-one, C(11)H(9)N(3)OS, (II), and 3-phenyl-6,7-dihydro-4H-1,3-thiazolo[2,3-c][1,2,4]triazin-4-one, C(11)H(9)N(3)OS, (III), which were prepared by the condensation reaction of (I) with 1,2-dibromoethane, have been characterized by X-ray crystallography and spectroscopic studies. The crystal structures of (II) and (III) both show two crystallographically independent molecules. While the two compounds are isomers, the unit-cell parameters and crystal packing are quite different and (II) has a chiral crystal structure.