The main aim of this work is the electronic characterization of various Co oxide and precursor states on Ag(001), which grow during reactive deposition by evaporating Co from a Knudsen cell in an O-2 atmosphere. We have identified double-layer CoO(001) and monolayer and double-layer CoO(111), which show distinct differences in the I/U and (dI/dU)/(I/U) characteristics of scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS). CoO(001) and, to some extent, also double-layer CoO(111) exhibit oxidic behavior. For CoO(001), the oxidic character is particularly recognized in the contrast dependence as a function of sample bias U. (C) 2004 Elsevier B.V. All rights reserved.