A new ab initio potential energy surface for the ground singlet state of the O-2 + O-2 system has been calculated. Good agreement with experiment is obtained for the van der Waals binding energy. The vibrational dependence of the potential, crucial for vibrational energy transfer studies, is analyzed and compared with other potential energy surfaces by performing quantum scattering calculations of the vibrational relaxation of O-2(v). (C) 2002 Elsevier Science B.V. All rights reserved.