We formalize the ordinal covalency scale for lanthanide compounds, introduced in a previous work, through an analytical function (the covalent fraction) of the overlap polarizability. The inverse of a parameter appearing in this function is interpreted as a typical average volume of the overlap region. The red shift in the nephelauxetic effect for the D-5(0) -> F-7(0) transition is analyzed, for 15 europium compounds, in terms of the ligand field strength parameter, N-v, and the covalent fraction. A clear tendency of this shift to increase with the covalent fraction is observed. A similar trend, though less clear, is also observed with respect to N-v, which is interpreted as a reflection of covalent effects in the ligand field parameters. (c) 2005 Elsevier B.V. All rights reserved.