The radon-water complex (Rn . H2O) is studied using ab initio methods employing effective core potentials combined with various valence basis sets. The minimum energy geometry and harmonic vibrational frequencies are calculated using levels of theory up to QCISD; the interaction energy is calculated using levels of theory up to the CCSD(T) level. The minimum energy geometry corresponds to a C-s hydrogen-bonded structure, with a near-linear Rn ... H-O bond, with a Rn ... H bond length of 3.0 Angstrom. A lower bound for the binding energy is obtained as D-e = 0.52 kcal mol(-1) (180 cm(-1)), which may be used to derive a Delta H-f(298) of -57.9 kcal mol(-1).