Ab initio and density functional investigations of HOBr-H2O and BrONO2-H2O complexes were performed. For HOBr-H2O complex, structures and vibrational frequencies were determined at MP2 and B3LYP levels of theory with basis sets up to 6-311++G(d,p), and binding energies were obtained from MP4/6-311++G(2df,2pd) single-point energy calculations. Two C-s symmetry conformers were found, and the binding energies including ZPE correction are 5.6 and 5.4 kcal mol(-1) for the syn and anti conformers, respectively. Electron correlation is important in determining the equilibrium structure of BrONO2-H2O complex which has a C-s equilibrium structure at HF/6-311G(d,p) level but slightly deviates from it at B3LYP/6-311G(d,p) level. The bromine atom acts as an electron acceptor in the complex. The ZPE corrected binding energy determined at B3LYP/6-311++G(2df,2p) level is 4.0 kcal mol(-1).