Work functions, ionization potentials (electron affinities) and Fermi level positions measured in-situ by photoelectron spectroscopy at surfaces of transparent conducting oxides are presented. Thin films of ZnO, ZnO:Al, SnO2, SnO2:Sb, In2O3, In2O3:Sn, and In2O3:(Zn,Sn) are prepared by magnetron sputtering. The Fermi level position is strongly affected by the oxygen content in the sputter gas. The ionization potential and work function of ZnO are strongly affected by surface orientation. In contrast, SnO2-based and In2O3-based materials show pronounced changes of ionization potential and work function induced by surface oxidation and reduction. Unlike SnO2, the oxidation of the In2O3-based TCO surfaces does not occur during deposition but can be induced by post-deposition treatments. (C) 2009 Elsevier B.V. All rights reserved.