We report the gas-phase detection and spectroscopic characterisation of ethynethiol (HCCSH), a metastable isomer of thioketene (H2C2S) using a combination of Fourier-transform microwave and submillimetre-wave spectroscopies. Several a-type transitions of the normal species were initially detected below 40 GHz using a supersonic expansion-electrical discharge source, and subsequent measurement of higher-frequency, b-type lines using double resonance provided accurate predictions in the

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... tre region. With these, searches using a millimetre-wave absorption spectrometer equipped with a radio frequency discharge source were conducted in the range 280–660 GHz, ultimately yielding nearly 100 transitions up to rR0(36) and rQ0(68). From the combined data set, all three rotational constants and centrifugal distortion terms up to the sextic order were determined to high accuracy, providing a reliable set of frequency predictions to the lower end of the THz band. Isotopic substitution has enabled both a determination of the molecular structure of HCCSH and, by inference, its formation pathway in our nozzle discharge source via the bimolecular radical-radical recombination reaction SH+C2H, which is calculated to be highly exothermic (−477 kJ/mol) using the HEAT345(Q) thermochemical scheme.