The non-adiabatic transitions between the first-tier E and D ion-pair states of iodine molecule induced by collisions with Ar atom are investigated using first-order semiclassical perturbation theory. PES's and diabatic couplings between electronic states are obtained in analytical form by the first-order perturbation theory-diatomics-in-molecule method. Electronic and vibrationally inelastic rate constants and product state distributions are calculated and compared with available experimental data. (C) 2003 Elsevier Science B.V. All rights reserved.