The emission of negative cluster ions in sputtering of silicon, germanium and silicon–germanium alloys was investigated by time of flight secondary ion mass spectrometry using Cs+ ions at 1 keV for sputtering and Ga+ ions at 25 keV for analysis. Various homonuclear (Si n-; Ge n-) and heteronuclear (SiGe n-; Si n Ge−) anionic cluster species were studied and their abundance distributions as a function of cluster size n were determined in steady state conditions. Surprisingly Si n-with n> 2 are less sensitive to Ge concentration than Si−. This implies that the variation of the secondary ion signals as a function of Ge concentration cannot be explained in terms of simple statistical considerations. Also Ge n-clusters show a peculiar behavior. Very strong matrix effects were observed for Ge− with decreasing intensity at high germanium concentrations. An opposite behavior is followed by the other clusters whose intensities …