The ground-state rotational spectra of five isotopic species, (C4H8S)-S-32 ... (HCl)-Cl-35, (C4H8S)-S-32 ... (HCl)-Cl-37, (C4H8S)-S-32 ... (DCl)-Cl-35, (C4H8S)-S-32 ... (DCl)-Cl-37, and (C4H8S)-S-34 ... (HCl)-Cl-35, of a hydrogen-bonded dimer between tetrahydrothiophene and hydrogen chloride have been measured in the frequency range 6-18.5 GHz using a molecular beam Fourier transform microwave spectrometer. Spectral analysis gave rotational, quartic centrifugal distortion and Cl-nuclear quadrupole coupling constants for each isotopomer. The rotational and quadrupole coupling constants have been interpreted in terms of a geometry in which hydrogen chloride lies on the plane bisector to the CSC angle of tetrahydrothiophene. The angle between the S ... Cl internuclear line and the line bisecting the CSC angle was found to be 86.6(7)degrees and the distance r(S ... Cl) = 3.48(3) Angstrom. The deviation of the atoms S ... H-Cl involved in the hydrogen bond from a collinear arrangement (theta = 0 degrees) was estimated to be theta -14 degrees.