The microwave spectrum of the water-carbonyl sulfide complex H2O-OCS was observed with a pulsed-beam, Fabry-Perot cavity Fourier-transform microwave spectrometer. In addition to the normal isotopic form, we also measured the spectra of H2O-(SCO)-C-13, H2O-(SCO)-S-34, (H2O)-O-18-SCO, D2O-SCO, D2O-(SCO)-C-13, D2O-(SCO)-S-34, HDO-SCO, HDO-(SCO)-C-13, and HDO-(SCO)-S-34. The rotational constants are B=1522.0115(2) MHz and C=1514.3302(2) MHz for H2O-SCO; B=1511.9153(5) MHz and C=1504.3346(5) MHz for H2O-(SCO)-C-13; B=1522.0215(3) MHz and C=1514.3409(3) MHz for H2O-(SCO)-S-34; B=1435.9571(3) MHz and C=1429.1296(4) MHz for (H2O)-O-18-SCO, B=1409.6575(5) MHz and C=1397.9555(5) MHz for D2O-SCO; B=1399.8956(3) MHz and C=1388.3543(3) MHz for D2O-(SCO)-C-13; B=1409.6741(24) MHz and C=1397.9775(24) MHz for D2O-(SCO)-S-34; (B+C)/2=1457.9101(2) MHz for HDO-SCO; (B+C)/2=1448.0564(4) MHz for HDO-(SCO)-C-13; and (B+C)/2=1457.9418(15) MHz for HDO-(SCO)-S-34, with uncertainties corresponding to one standard deviation. The observed rotational constants for the sulfur-34 complexes are generally higher than those for the corresponding sulfur-32 isotopomers. The heavier isotopomers have smaller effective moments of inertia due to the smaller vibrational amplitude of the S-34-C vibration (zero point) as compared to the S-32-C, making the effective O-S-34 bond slightly shorter. Stark effect measurements for H2O-SCO give a dipole moment of 8.875(9)x10(-30) C m [2.6679(28) D]. The most probable structure of H2O-SCO is near C-2v planar with the oxygen of water bonded to the sulfur of carbonyl sulfide. The oxygen-sulfur van der Waals bond length is determined to be 3.138(17) Angstrom, which is very close to the ab initio value of 3.144 Angstrom. The structures of the isoelectronic complexes H2O-SCO, H2O-CS2, H2O-CO2, and H2O-N2O are compared. The first two are linear and the others are T shaped with an O-C/O-N van der Waals bond, i.e., the oxygen of water bonds to the carbon and nitrogen of CO2 and N2O, respectively. (C) 2004 American Institute of Physics.