The HOBr - H2O concentration at different altitudes is important to understand the contribution of the respective monomer species for the catalytic cycles of reactions involving ozone depletion. To calculate the concentration profile of the hydrated complex, thermodynamic properties were determined based on the calculated MP2/6-311 ++G(2d,2p) equilibrium geometry, rotational constant, vibrational frequencies and CCSD(T)/6-311 ++G(3df,3pd) BSSE corrected energy values. The equilibrium constant and concentration profile were determined at different altitudes for the syn and anti dimers. It was found that the HOW is present at higher altitudes mainly as free species, being available to participate in the reactions involved in the ozone destruction. (c) 2005 Elsevier B.V. All rights reserved.