We report the results of an ab initio study of the HOCl-H2O complex. Structures and harmonic vibrational frequencies are determined at the second-order Moller-Plesset level of theory with basis sets up to 6-311S++G-(d,p) in size, and energies were determined from MP4/6-311++G(3df,3pd) single-point calculations. Two conformers of a hydrogen-bonded complex were obtained; HOCl acts as the proton donor in both. Binding energies at 0 K (corrected for zero-point vibrational energy) were 5.6 and 5.9 kcal mel(-1). Calculated shifts in monomer vibrational frequencies upon complexation agree well with those obtained previously in a matrix isolation study of the complex.