Two new polymorph forms, (Ia) and (Ib), of the title compound, C(14)H(17)N(3)S, and its solvate with acetonitrile, C(14)H(17)N(3)S.0.25C(2)H(3)N, (Ic), have been investigated. Crystals of the two polymorphs were grown from different solvents, viz. ethanol and N,N-dimethylformamide, respectively. The polymorphs have different orientations of the thioamide group relative to the CN substituent, with s-cis and s-trans geometry of the C=C-C=S diene fragment, respectively. Compound (Ic) contains two independent molecules, A and B, with s-cis geometry, and the solvate molecule lies on a twofold axis. The core of each molecule is slightly non-planar; the dihedral angles between the conjugated C=C-CN linkage and the phenyl ring, and between this linkage and the thioamide group are 13.4 (2) and 12.0 (2) degrees in (Ia), 14.0 (2) and 18.2 (2) degrees in (Ib), 2.3 (3) and 12.7 (4) degrees in molecule A of (Ic), and 23.2 (3) and 8.1 (4) degrees in molecule B of (Ic). As a result of strong conjugation between donor and acceptor parts, the substituted phenyl rings have noticeable quinoid character. In (Ib), there exists a very strong intramolecular steric interaction (H...H = 1.95 A) between the bridging and thioamide parts of the molecule, which makes such a form less stable. In the crystal structure of (Ia), intermolecular N-H...N and N-H...S hydrogen bonds link molecules into infinite tapes along the [1-10] direction. In (Ib), such intermolecular hydrogen bonds link molecules into infinite (101) planes. In (Ic), intermolecular N-H...N hydrogen bonds link molecules A and B into dimers, which are connected via N-H...S hydrogen bonds and form infinite chains along the c direction.