The rotational spectrum of etheneselenol was investigated in the microwave and millimeter-wave ranges. Two conformers of comparable stability were identified: the syn conformer, which is planar, and the gauche conformer. The experimental study has been coupled with some ab initio calculations, which greatly helped to the identification of the spectra. The etheneselenol-d (Se-D) was also investigated and the quadrupole coupling constants of deuterium determined.