Cyanoethenethiol (NC-CH=CH-SH) was obtained in a 8:1 Z:E ratio by flash vacuum thermolysis of the t-butylsulfide derivative. Density functional theory (DFT) and G3 ab initio calculations predict the existence of Z-and E-isomers, each of which exhibits two rotamers as a function of the relative position of the SH group. All these rotameric forms are planar (C(s) symmetry) and correspond to synperiplanar and antiperiplanar conformations. Calculations indicate that the synperiplanar Z-isomer is the more stable. In pure form, the cyanoethenethiol rapidly decomposes at room temperature, even at low pressure and partially condenses on the wall of the cell. To record its spectrum, a long optical path of 136 m was necessary, and several successive fillings of the cell were required. On the basis of the calculated harmonic and anharmonic vibrational frequencies, a complete and unambiguous assignment of the experimental spectrum has been carried out.