The microwave spectra of ethaneselenol and its deuterated and 13C-substituted species were measured and assigned for the gauche and trans isomers. The double minimum splittings in the gauche isomers were directly observed for the species having a symmetry plane in the frame part. The rotational constants and the torsional splitting of the gauche isomer of the parent species were determined to be A = 27 148.86 ± 0.05, B = 3 623.68 ± 0.01, C = 3 399.21 ± 0.03, and ∆ ν = 1 083.33 ± 0.04 MHz. From the torsional splittings of the parent and SeD species together with the vibrational frequencies already reported by Durig and Bucy, the Fourier coefficients of the selenol internal rotation potential function were determined to be V1 = -44 ± 17, V2 = -260 ± 3, V3 = 1202 ± 16, and V6 = -43 ± 9 cal/mole. From the rotational constants obtained, the rs structural parameters of the gauche and trans isomers were determined. The structural parameters in the skeletal part for the gauche isomer are r (CC) = 1.524 Å, r (CSe) = 1.957 Å, r (SeH) = 1.467 Å, α(CCSe) = 113°31', α(CSeH) = 93°05', and the dihedral angle τ(CCSeH) = 61°39'. Those for the trans isomer are r (CC) = 1.525 Å, r (CSe) = 1.962 Å, r (SeH) = 1.440 Å, α(CCSe) = 108°43', and α(CSeH) = 93°30'. These parameters were compared with the corresponding ones of ethanethiol.