The weakly bound trimer between two water molecules and one hydrogen chloride molecule was identified from its rotational spectrum observed at conditions of supersonic expansion. The spectrum was measured with a cavity, Fourier transform microwave spectrometer and the rotational, quartic, and chlorine hyperfine coupling constants were determined for six isotopomers of the complex: ((H2O)-O-16)(2)(HCl)-Cl-35, ((H2O)-O-16)(2)(HCl)-Cl-37, ((HOD)-O-16 ...(OH2)-O-16)(HCl)-Cl-35, ((HOH)-O-18 ...(OH2)-O-16)(HCl)-Cl-35, ((HOH)-O-16 ...(OH2)-O-18)(HCl)-Cl-35, and ((H2O)-O-18)(2)(HCl)-Cl-35. The rotational spectrum consists of transitions allowed by the mu(a) and the, much smaller, mu(b) dipole moment components. The rotational transitions are further split into components belonging to four low-lying vibration-rotation-tunneling substrates. The trimer is bound strongly enough to attenuate the dynamics of the water subunits to a level such that the differences in rotational constants between the four states are below 1 MHz. The complex is near planar as evidenced by inertial defect of -0.5 u Angstrom(2). Analysis of experimental and theoretical results reveals that the three monomers are bound in a triangular arrangement through O ... HO and O ... HCl hydrogen bonds and a primarily dispersive OH ... Cl bond. All atoms, with the exception of two nonbonded hydrogens, lie near a common plane. The r(s), r(0), and r* structural parameters have been evaluated and the two hydrogen bonds r*(OH ... O) = 2.8151(8), and r*(O ... HCl) = 3.0840(11) Angstrom are appreciably shorter than analogous bonds in (H2O)(2) and H2O ... HCl, whereas r*(OH ... Cl) = 3.4152(12) Angstrom is close to the sum of the van der Waals radii for O and Cl. The experimental results concerning all observables determinable from the rotational spectrum are found to be in very good agreement with theoretical predictions. (C) 2000 American Institute of Physics. [S0021-9606(00)02013-4].