M-shell excitations in Rare-Earth (RE) materials were studied using synchrotron radiation and total electron yield detection in UHV. In mixed-valent compounds of the heavy RE's (Sm, Eu, Tm), the M4,5 spectra are identified as superpositions of quasi-atomic 3d94fn+1 multiplet structures, providing quantitative information on the mean valence v̄ (4f occupation) of the RE ions, even in highly dilute systems. The method employed in deriving quantitative values for v̄ is described in some detail. Effects of 4f/valence-band hybridization on the M4,5 spectra are investigated for Ce systems: the 3d94f2 multiplet structure seen in γ-like Ce compounds is found to collapse in α-like systems, where satellites at about 5 eV higher energies are additionally observed. The method is also applied to dilute Ce systems, such as Ce in a Pd matrix, which is found to be α-like. In addition, M3-edge spectra are presented for a series of Ce compounds, and their relation to L3-edge spectra is discussed. © 1985.