Tunneling spectroscopy and voltage-dependent scanning tunneling microscopy have been used to study the geometry and electronic properties of atomic-sized defects on the Si (001) surface. Individual dimer vacancies are shown to be semiconducting, consistent with the π-bonded defect model of Pandey. Another type of characteristic defect is found which gives rise to strongly metallic tunneling I-V characteristics, demonstrating that it has a high density of states at the Fermi level and is likely active in Fermi level pinning on Si (001). Spatially dependent I-V measurements and tunneling barrier height measurements also directly reveal the spatial extent of this metallic character and provide direct measures of the “size” of the defects. © 1989, American Vacuum Society. All rights reserved.