Tunneling microscopy and spectroscopy are used to characterize defects in Al overlayers on Si(111). At low coverages where Al and Si adatoms are both present, voltage‐dependent scanning tunneling microscopy imaging allows them to be selectively imaged due to their different electronic structures. In (3)1/2‐Al, Si adatoms substituting for Al give rise to a defect state at −0.4 eV which is strongly localized in space. Band‐bending measurements indicate that the Si substitutional defects are electrically neutral. The results are interpreted in terms of a Mott–Hubbard model for a strongly correlated system.