The donor-acceptor complex formed from NH3 and SO3 has been observed in the gas phase by pulsed nozzle Fourier transform microwave spectroscopy. The adduct has the expected C(3v), geometry in which the nitrogen is bonded to the sulfur. The N-S bond length is 1.957(23) Å and the NSO angle is 97.6(4)Δ. The complex is the gas-phase form of the zwitterion which constitutes solid sulfamic acid, but the observed structure differs significantly from the results of previous X-ray and neutron diffraction work on the crystal. In particular, the N-S distance is 0.186(23) Å longer in the gas phase and the NSO angle is 4.9(4)Δ smaller. The observed structure suggests that the dative bond is only partially formed in the isolated complex. Analysis of the nitrogen quadrupole coupling constant supports this conclusion, indicating that about 0.36 electron is transferred upon formation of the complex. The nature of the gas-phase adduct suggests that significant structural and electronic changes may occur within +H3N-SO3- monomer units during the early phases of homogeneous nucleation.