Abstract The intensity variations of atomic and molecular secondary ions emitted from cesiumbombarded silicon have been investigated at impact energies between 2 and 12 keV. Accumulation of implanted cesium was found to enhance the Si − intensity by up to five orders of magnitude. Simultaneously, Cs + emission is supressed by more than two orders of magnitude. During cesium implantation the energy distribution of Si − (Cs +) shifts towards lower (higher) energies. The Si − (Cs +) intensity increases (decreases) with decreasing impact energy. Measurements involving Si n − cluster ions suggest that the stationary surface concentration of cesium increases with decreasing impact energy. The observed phenomena are apparently affected by even weak electric fields at the sample surface and by bombardment-induced diffusion. Nevertheless the work function appears to be the key parameter governing both negative and positive secondary ion yields.