We have measured the angular distribution of H, molecules with incident energies 100-150 meV reflected by the Pd(1 1 1) surface at 430 K. In spite of the low diffraction reflectivity (similar to0.5%), diffraction peaks have been resolved at nearly grazing incidence. Apart from specular reflection. out-of-plane diffraction has been found to be dominant, in agreement with classical simulations. We show that this behavior is a generic one for grazing incidence and discuss the relevance of this finding for the study of molecule/surface dynamics in systems for which dissociation is a non activated process. (C) 2004 Elsevier B.V. All rights reserved.