The structure and stability of the molecular clusters NH3.SO3...(H2O)n, n = 1-6, 9, 12, are studied using density functional theory. The results show that the electron donor/acceptor complex NH3.SO3 is stable with water and has an unusually high affinity for incoming water molecules. The complex itself is progressively stabilized by the addition of water molecules, as indicated by the shortening of the N-S bond distance with an increasing number of water molecules. The binding energy of the cluster to each H2O molecule is about 12 kcal mol(-1), two to three times the binding energy of a pure water dimer, and it remains approximately constant as the cluster increases in size with an increasing number of H2O molecules. The N-S distance decreases monotonically with the addition of each H2O molecule, indicating that the NH3.SO3 unit in the clusters is stabilized by the introduced H2O. The calculated group charge on SO3 (or NH3) increases as the N-S distance successively decreases, which reveals that the stability of the clusters originates from mutually enhanced electrostatic interactions between NH3.SO3 and H2O. This study strongly suggests that NH3.SO3 may act as an effective nucleation agent for the formation of atmospheric aerosols and cloud particles.