The rotational spectra of three isotopic species C4H6O ... (HCl)-Cl-35, C4H6O ... (HCl)-Cl-37 and C4H6O ... (DCl)-Cl-35 of the hydrogen-bonded dimer formed by cyclobutanone and hydrogen chloride have been studied by means of microwave spectroscopy in a supersonic jet. Rotational constants, centrifugal distortion constants and Cl-nuclear quadrupole coupling parameters have been determined for the ground state and for a vibrationally excited state attributable to the ring-puckering motion of the cyclobutanone subunit. The spectroscopic constants have been interpreted in terms of a C-s symmetry in which the monomers are coplanar with: r(O ... H) = 1.86(3) Angstrom, angle(C=O ... H) = 112.3(7)degrees and a deviation theta of the O ... H-Cl nuclei from collinearity of 13.1(6)degrees.