The methanol dissociative adsorption on Pt(100) at low potentials in the hydrogen region is studied using nonlinear vibrational sum-frequency generation (SFG) spectroscopy. It is shown that adsorbed hydrogen atoms effectively block atop sites at the surface. In the hydrogen potential region, CO produced by the methanol dissociation adsorbs at step-edges and in bridge sites on terraces as shown by the characteristic stretching frequencies observed for adsorbed CO. (C) 2003 Elsevier B.V. All rights reserved.