The hydrogen-bonded complex oxetane . . . HCl generated ill a supersonic jet has been characterised by Fourier transform microwave spectroscopy. For the observed conformer the spectra of the four isotopic species C3H6O . . .(HCl)-Cl-35, C3H6O . . .(HCl)-Cl-37, C3H6O . . .(DCl)-Cl-35 and C3H6O . . .(DCl)-Cl-37 have been measured. Rotational data are consistent with a C-s symmetry for the complex, with HCl lying in the symmetry plane bisector to the COC angle. The planar configuration of isolated oxetane has been reasonably elucidated to remain in the dimer, which implies that no significant change in the potential energy function of ring puckering motion of oxetane occurs upon complexation.