Ground-state rotational spectra of the seven isotopomers H2O . . . Cl-35(2), H2O . . . (ClCl)-Cl-37-Cl-35, H2O . . . (ClCl)-Cl-35-Cl-37, D2O . . . Cl-35(2), D2O . . . (ClCl)-Cl-37-Cl-35, HDO . . . Cl-35(2), and HDO . . . (ClCl)-Cl-37-Cl-35 of the complex formed between water and dichlorine were observed by pulsed-jet, Fourier-transform microwave spectroscopy. Rotational constants B-0 and C-0, centrifugal distortion constants Delta (J) and Delta (JK), and Cl nuclear quadrupole coupling constants chi (aa)(Cl-x) and {chi (bb)(Cl-x)-chi (cc)(Cl-x)}, where x=i (inner) or o (outer), were obtained via spectral analyses. The spectroscopic constants were interpreted, on the basis of models for the complex, to give the geometry, the binding strength, and the extent of inter-and intramolecular electron transfer on complex formation. The zero-point geometry is defined in terms of the effective values r(O . . . Cl-i)=2.8479(3) Angstrom and phi =43.4(3)degrees, where phi is the angle made by the bisector of the HOH angle with the O . . . Cl internuclear line. The intermolecular stretching force constant k(sigma)=8.0(1) N m(-1). Fractions delta (i)=0.005(5) and delta (p)=0.034(3) of an electron were estimated to be transferred from O to Cl and from Cl-i to Cl-o, respectively. The geometrical conclusions are supported by ab initio calculations at the aug-cc-pVDZ/MP2 level of theory, with good agreement for phi and r(O . . . Cl-i). A comparison of the properties of H2O . . . Cl-2 with those of H2O . . . HCl provides evidence in support of a recent proposal of a halogen bond B . . . XY that is the analog of the hydrogen bond B . . . HX, where X and Y are halogen atoms.