The interactions and reactions of sulfur trioxide (SO3), ammonia (NH3), and water (H2O) are investigated using density functional theory and ab initio molecular orbital theory. Four stable clusters that result from strong intermolecular interactions and reactions of the ternary SO3/NH3/H2O system are considered: SO3.H2O . . . NH3, SO3. NH3. . .H2O, H2SO4. . . NH3, and HSO3NH2. . .H2O. The sulfuric acid-ammonia cluster, H2SO4. . . NH3. is found to be the most stable and represents the ultimate fate of the ternary system. However, the zwitterionic sulfamic acid-water cluster, SO3. NH3. . . H2O, is almost equally stable, indicating that the SO3. NH3 complex may be thermodynamically stable in water vapor. The reaction pathways are studied for the interconversions between the four clusters. Large energy barriers are found between SO3. NH3. . .H2O and SO3.H2O . . . NH3 and between SO3. NH3. . .H2O and HSO3NH2. . .H2O, and a small barrier is found between SO3.H2O . . . NH3 and H2SO4. . . NH3. As a result, SO3.H2O . . . NH3 readily converts into the sulfuric acid cluster, H2SO4. . . NH3, but conversion of SO3. NH3. . .H2O into either SO3. . .H2O . NH3 or HSO3NH2. . .H2O is kinetically unfavorable. The results suggest that the intermediate fate of SO in the atmosphere depends on the relative concentrations of H2O and NH3. In normal atmospheric conditions, where the SO3.H2O complex forms first due to overwhelmingly larger H2O mixing ratio, the addition of NH3 to SO3.H2O is likely to form H2SO4. . . NH3 which evolves into a nucleus of sulfate-based aerosol. On the other hand, in atmospheric conditions where an unusually high NH3 mixing ratio exists, the SO3. NH3 complex may form first and further stabilize with H2O.