The microwave spectrum of the nitromethane-water complex (\left(\mathrm{CH}{3} \mathrm{NO}{2}-\mathrm{H}{2} \mathrm{O}\right)) has been studied with a pulsed-beam Fourier-transform Fabry-Perot-cavity spectrometer. Both (a)-type and (h)-type transitions were observed for the (A) state of the complex with the (h)-type transition being more intense by a factor of 2. Critical to the rotational assignments were well resolved ({ }^{14} \mathrm{~N}) nuclear electric quadrupole transitions, and the incorporation of the pulsed nozzle in one of the mirrors which provided a beam coaxial with the cavity axis to attain linewidths of about (2 \mathrm{kHz}) (fullwidth at half maximum-FWHM). To provide additional structural information, the spectra of the HDO, (\mathrm{D}{2} \mathrm{O}). and (\mathrm{CD}{3} \mathrm{NO}{2}) substituents were assigned. The molecular structure derived from the moments of inertia has a center of mass separation of (3.506(7) \AA). The moments of inertia can not distinguish between two possible forms of the complex, one with the dipole moment vectors aligned and the other with them antialigned. Measurement of the dipole moment components, (\mu_{a}=4.15(4) \times 10^{-30} \mathrm{C} \cdot \mathrm{m}\left(\mu_{a}=1.243(11) \mathrm{D}\right)) and (\mu_{b}=5.96(4) \times 10^{30} \mathrm{C} \cdot \mathrm{m}\left(\mu_{b}=\right.) (1.706(12)) D). resolves this difficulty and shows that the dipole moment vectors are antialigned. The molecular complex is quite strongly bound with a harmonic pseudodiatomic stretching force constant. (k_{\mathrm{s}}=9.30 \mathrm{~N} / \mathrm{m}). and hydrogen bond lengths of approximately (2.1 \AA) between the water (\mathrm{O}) atom and a (\mathrm{CH}{3} \mathrm{H}) atom, and between a water (\mathrm{H}) atom and an oxgen atom of (\mathrm{NO}{2}). One standard deviation uncertainty is shown in parentheses for each experimental value reported. c) 1995 Academic Press. Inc.