We have studied the formation of the H-2 molecule on a graphite surface, when both H atoms are initially physisorbed. The graphite surface is assumed to be planar, and a model potential is obtained in a semiempirical way to reproduce the experimental properties of H physisorption on graphite. The reaction probability has been computed in the case when the angular momentum of the relative H-H motion lies parallel to the surface plane. Three-dimensional wave packet calculations have been performed for collision energies ranging from 2 to 50 meV. It is shown that the reaction occurs with a significant probability and produces the H-2 molecule with a considerable amount of vibrationnal energy. A simple mechanical model is presented, where desorption of the nascent H-2 molecule results from two successive binary elastic collisions. (C) 2004 American Institute of Physics.