We determine the effects of having vacancies on the Pt(111) surface to the dissociative adsorption behavior of H₂ on Pt(111). We study the potential energy behaviors along the reaction paths based on the potential energy surfaces (PESs) we calculated that are relevant to these systems. By comparing these results to those of an ideal Pt(111) surface, we find that the vacant sites enhance the surface reactivity by lowering the activation barriers for H₂ dissociative adsorption, as shown by several new reactions paths we observed corresponding to near spontaneous reactions. The results also corroborates our earlier findings that of the substrate atoms, the surface (first-layer) atoms mainly influence the behavior of H₂ adsorption on Pt(111), and that the onset of H₂ dissociation and barrier location are also altered by the presence of vacancies. [DOI: 10.1380/ejssnt.2008.134]