A prototropic rearrangement of N-prop-2-enylbenzocarbothioamide afforded two stereoisomeric N-prop-1-enylbenzocarbothioamides (IIa, b) isolated as individual compounds. The stereoisomerism is due to hindered rotation around thioamide bond C(S)-N and also to pi-diastereoisomerism. The configuration of the isolated isomers was investigated by IR spectroscopy, AM1 and PM3 methods. In the molecules occurs the p,pi-conjugation with participation of nitrogen atom and multiple bonds resulting in formation of a united delocalized MO in the planar fragment C(S)-NC=C.