We investigate the relaxation of highly vibrationally excited HO2 in collisions with vibrationally cold O-2. The calculations employ the quasiclassical trajectory method and a global double many-body expansion potential energy surface for HO4((2)A). Both deactivation and activation processes are observed, with the rate constants being found to be strongly influenced by the donor internal energy content. It is also found that the title process may not be ignorable when modelling the ozone budget in the middle atmosphere. (C) 2004 Elsevier B.V. All rights reserved.