The complexes of ammonia with sulfur trioxide have been studied using FTIR matrix isolation spectroscopy and DFT/B3LYP calculations with the aug-cc-pVTZ basis set. The NH3/SO3/Ar matrixes were prepared in two different ways. In one set of experiments the matrix was prepared by the simultaneous deposition of the NH3/Ar mixture and SO3 vapor from the thermal decomposition of K2S2O7. In the second set of experiments thermolysis products of sulfamic acid were trapped in an argon matrix. Both methods of matrix preparation led to the formation of the H3N center dot SO3 electron donor-acceptor complex that was characterized earlier. In the matrixes comprising therrnolysis products of sulfamic acid, in addition to H3N center dot SO3, the H3N-SO3 center dot center dot center dot NH3 complex (IID) was also identified. The identity of the complex was confirmed by comparison of the experimental and theoretical spectra of H3N-SO3 center dot center dot center dot NH3 and D3N-SO3 center dot center dot center dot ND3. The performed calculations show that in H3N-SO3 center dot center dot center dot NH3 the two N atoms and the S atom are collinear; the two S-N bonds are nonequivalent, one is much shorter (2.230 angstrom) than the other one (2.852 angstrom). In the AIM topological analysis, the interaction energy decomposition and topological properties of the electron localizability indicator (ELI-D) allowed us to categorize the stronger N-S bond in the IID complex as a dative bond and to assume that the fragile N-S bond is a consequence of a weak electron-donor-acceptor interaction. The calculations indicate that the identified IID complex corresponds to a local minimum on the PES of the NH3/SO3 system of 2:1 stoichiometry. The (NH3)(2)SO3 complex, IIHB, corresponding to a global minimum is 7.95 kcal mol(-1) more stable than the IID complex. The reason that the IID complex is present in the matrix but not the IIHB complex is discussed.