Density functional theory with 6-311++G** basis sets has been applied to the investigation of intermolecular interactions of sulfur trioxide and sulfuric acid dimers. Counterpoise procedure was used for the basis set superposition error (BSSE) correction. The corrected binding energy for (HSO4) 2 is -71.37 kJ/mol at the B3LYP/6-311++G** level, and -37.79 and -39.04 kJ/mol for two H2SO4/SO3 binary complexes (denoted as dimers B and C) respectively. Dimers B and C can be spontaneously produced when gaseous sulfur trioxide mixes with the sulfuric acid, which is in agreement with the experimental fact that the sulfur trioxide is prone to dissolve into the sulfuric acid. The O…S interacting is stronger than the O…H interacting in the heterodimer of H2SO4/SO3. Electron density at bond critical points and natural bond orbital analysis were performed to probe the origin of the intermolecular interactions.