The microwave spectra of five isotopically substituted derivatives of H2O-SO3 have been observed by pulsed nozzle Fourier transform microwave spectroscopy. The complex, which has long been regarded as an important precursor to H2SO4 in the atmosphere, has a structure in which the oxygen of the water approaches the sulfur of the SO3 above its plane, reminiscent of a donor-acceptor complex. The intermolecular S-O bond length is long (2.432 ± 0.003 Å), and the out-of-plane distortion of the SO3 is small (2-3°). The C2 axis of the water forms an angle of 103 ± 2° with the intermolecular bond. For an eclipsed configuration, this structure places the protons 2.67 Å from the SO3 oxygens, indicating that a rather long distance must be traversed in order to transfer a proton to form sulfuric acid. The success of these experiments depended critically on the use of a molecular source in which liquid water was evaporated directly into the supersonic expansion. Such a source should be general for liquids of moderate vapor pressure, and its design is described.