The emission of negative cluster ions in sputtering Si, Ge and their oxides has been investigated by time of flight secondary ion mass spectrometry using Cs+ ions at different energies for sputtering and Ga+ at 25 keV for analysis. In GeO2 the most intense signals come from the O- (n <= 2) and GeOn- (=) (2,3) clusters. The mass spectrum of GeO2 is therefore very similar to that Of SiO2 where the most intense signals are related to the O- (n <= 2) and SiOn- (=) (2,3) clusters. A full spectrum approach has been applied to study the variation of the ionization probability in the different matrices when reducing the Cs+ ion energy, i.e. increasing the cesium concentration at the surface. In all the matrices, the total number of atoms in the ionized fraction of the sputtered flux increases when increasing the cesium content at the surface. Despite these variations in GeO2 (SiO2) the fractions of germanium (silicon) and oxygen in the ionized part of the sputtered flux are independent of the cesium content in the matrix. Moreover, the fractions of silicon and germanium in the two oxides are equivalent. (c) 2006 Elsevier B.V. All rights reserved.