The 1:1 molecular adduct of propylene oxide and water (PO-H2O) was studied using Fourier transform microwave spectroscopy and high level ab initio methods. Two distinct structural conformers with the water molecule acting as a proton donor were detected experimentally: one with the water on the same side as the methyl group with respect to the ether ring, i. e., syn-PO-H2O, the other with the water molecule binding to the O-atom from the opposite side of the methyl group, i. e., anti-PO-H2O. The nonbonded hydrogen is entgegen to the ether ring in both conformers. Rotational spectra of four isotopic species, namely PO-H2O, PO-DOH, PO-HOD, and PO-D2O, were recorded for the two conformers. The hydrogen bond parameters: r((OepoxH)-H-...), angle( ring-(OepoxyH)-H-...), and angle((OepoxyH)-H-...-O) are 1.908 angstrom, 112 degrees, and 177 degrees for syn-PO-(HO)-O-2, and 1.885 angstrom, 104.3 degrees, and 161.7 degrees for anti-PO-H2O, respectively. The experimental results suggest that the hydrogen bond in syn-PO-H2O is stronger and the monomer subunits are more rigidly locked in their positions than in the ethylene oxide- water adduct. The stabilizing effect of the methyl group to the intermolecular hydrogen bond is discussed in terms of the experimentally estimated binding energies, the structural parameters, and the ab initio calculations.