This paper describes the surface analyses of "in situ" prepared Mo cathodes with a La coating. It was found that the electron emission depends strongly on the atomic La/O ratio at the surface: the higher this ratio, the better the emission. The emission current density increases sharply when the La/O ratio starts to deviate from the stoichiometric ratio of La2O3. A model, which describes the emission of La coated Mo cathodes qualitatively, is presented. (c) 2005 Elsevier B.V. All rights reserved.