The crystal structures of three 3-methyl-1H-1,2,4-triazole-5-thio­nes, including a second polymorph of 4-[(E)-(5-bromo-2-hy­droxy­benzyl­idene)amino]-3-methyl-1H-1,2,4-triazole-5(4H)-thione and a redetermination of 4-amino-3-methyl-1H-1,2,4-triazole-5(4H)-thione

The structures of three 3-methyl-1H-1,2,4-triazole-5-thione derivatives are reported. The structure of 4-amino-3-methyl-1H-1,2,4-triazole-5(4H)-thione, C(3)H(6)N(4)S, (I), has been redetermined with an improved model for the H atoms: the non-H atoms of (I) all lie on mirror planes in space group Pbcm, and the H atoms of the methyl group are disordered over two sets of reflection-related atomic sites having occupancy 0.5: two independent N—H⋯S hydrogen bonds link the mol­ecules of compound (I) into complex sheets. The non-H atoms in the mol­ecules of 4-[(E)-(3,4-di­meth­oxy­benzyl­idene)amino]-3-methyl-1H-1,2,4-tri­azol-5(4H)-thione, C(12)H(14)N(4)O(2)S, (II), despite lying in general positions are close to planar, with a dihedral angle between the two rings of 6.31 (10)°: the mol­ecules of compound (II) are linked by a three-centre N—H⋯(O)(2) hydrogen bond into a C(10)C(11)[R (1) (2)(5)] chain of rings. A second polymorph of 4-[(E)-(5-bromo-2-hy­droxy-5-bromo­benzyl­idene)amino]-3-methyl-1H-1,2,4-triazole-5(4H)-thione, C(10)H(9)BrN(4)OS, (III), has been identified; the non-H atoms are nearly co-planar with a dihedral angle between the two rings of 1.9 (4)°. There is an intra­molecular O—H⋯N hydrogen bond and the mol­ecules are linked by N—H⋯S hydrogen bonds, forming centrosymmetric R (2) (2)(8) dimers. Comparisons are made with some related structures.