Optimal structures, interaction energies, and harmonic vibrational frequencies of the NH3 ... H2O complex have been determined from the second-order Moller-Plesset perturbation theory and DFT/B3PW91 calculations with the aug-cc-pVTZ basis set. Some properties of the complex have been calculated at the MP4 and CCSD(T) levels. The tunneling motion of the water molecule around its c inertial axis was studied, and the barrier to the exchange of the bound and free hydrogen atoms was determined as 1267 cm(-1). The nature of the intermolecular interactions in the complex was investigated by symmetry-adapted perturbation theory (SAPT). As revealed by the SAPT analysis the main binding contributions are the electrostatic and induction components. The calculations of the vibrational frequencies and infrared intensities for this complex an presented to facilitate the frequency assignments of the experimental spectra.