Ab initio configuration interaction (CI) calculations have been performed for the O2 + H2 complex in a trapezoidlike collision arrangement with C2v symmetry. The potential energy surfaces of the four lowest states of this van der Waals complex (arising from the X3 Σg−, a1 Δg, and b1 Σg+ states of the oxygen moiety), as well as the collision-induced b1 Σg+ − a1 Δg electric dipole transition moment (Mb–a), have been analyzed for different CI expansions, using as a reference determinant the restricted open-shell Hartree–Fock (ROHF) function for the ground state of the complex H2(X1 Σg+) + O2(X3 Σg−). The geometry optimized at the ROHF/6–311G** level was refined by a partial optimization at the CI level scanning the intermolecular distance. The equilibrium distances for the X, a, and b states have been found to be a slightly different in the region 3.02–2.98 Å. The larger binding energy of the b1 Σg+ state (2.96 kJ/mol) in comparison with the a1 Δg (2.1 kJ/mol) and ground X3 Σg− states (1.35 kJ/mol) presumably could be explained as resulting from charge-transfer interactions. A good convergence of the calculated transition moment Mb–a for the larger CI expansions (approximately 50,000 configuration-state functions) has been obtained. The calculated collision-induced intensity of the b1 Σg+-a1 Δg and a1 Δg−X3 Σg− transitions in molecular oxygen are in reasonable agreement with recent experimental data for several foreign gases. © 1994 John Wiley & Sons, Inc.