The potential energy surface for the H-2-I-2 system was determined in analytic variational electronic structure calculations with single- and double-substitution configuration interactions for the 16 valence electrons and with effective potentials for the iodine core electrons. The favored pathway for the overall reaction H-2 + I-2 --> HI + HI was found to pass through the region of the collinear configuration I-H-H-I. The pathway is accessible to bound and unbound iodine atom pairs and it allows the bimolecular and termolecular reactions, H-2 + I-2 --> HI + HI and H-2 + I +I --> HI + HI, both proposed by Bodenstein 100 years ago.