A wavepacket study of the dynamics of H-2 formation through a collinear Eley Rideal mechanism that explicitly handles the substrate relaxation effects is presented. The substrate used is a planar polycyclic aromatic hydrocarbon (PAH) which exhibits some strong similarities with a perfect graphite (0 0 0 1) surface. The collision energy range investigated lies between 0.4 meV (T = 5 K) and 0.46 eV. The reaction probability is large, except at very low collision energy. Most of the available energy goes into H, vibration. The substrate relaxation facilitates the reaction. (C) 2003 Published by Elsevier B.V.