Three well-defined two-dimensional (2D) Al overlayers including the Al/Si(111) R-phase with Al coverage of ∼0.25 ML, the Al/Si(111) γ-phase with Al coverage of ∼0.8 ML, and the bulk Al film with Al coverage of 16 ML on Si(111) have been prepared using the molecular beam epitaxy technique. Their surface morphology and electronic structure were characterized by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). The 16 ML Al film presents the metallic Al character, while the nanostructured Al/Si(111) R-phase and γ-phase are semiconducting due to space confinement of the surface Al atoms. The interfacial Al-Si interaction results in electron deficiency in surface Al atoms which follows the order of Al/Si(111) R-phase > Al/Si(111) γ-phase > bulk Al film. Methanol dissociation reactions on the Al surfaces at room temperature were studied by XPS and high-resolution electron energy loss spectroscopy (HREELS). The Al/Si(111) R-phase presents the highest activity for the dissociation of CH 3 OH and CH 3 O ads and the lowest activity for the bulk Al film. The exceptional activity of the nanostructured 2D Al surfaces for O-H and C-O bond scission has been attributed to their unique geometric structures as well as their semiconducting and electron-deficient characters.