Rotational spectra of vinyl mercaptan (ethenethiol) CH 2CHSH and its isotopic modification CH 2CHSD have been studied by microwave spectroscpy. The molecule has been found to exist in two rotameric forms, syn and anti, associated with different orientations of the SH bond with respect to the vinyl framework. In this paper results are reported for the more stable syn form which is shown to be planar with ground state rotational constants A = 49 815.28(6) MHz, B = 5835.716(14) MHz, C = 5222.081(11) MHz, DJ = 2.85(17) kHz, DJK = -33.22(2.08) kHz, and δJ = 0.425(65) kHz. Spectra have also been observed for the first and second excited states of the SH torsional vibration and the first excited state of the CCS angle bending mode. The dipole moment of the syn rotamer is μa = 0.813(1), μb = 0.376(4), and μtotal = 0.896(3) D.