This paper summarizes some of the recent advances in the understanding of the secondary ion emission process, and relates them to the important cesium and oxygen enhancement effects in secondary ion mass spectrometry (SIMS). The oxygen enhancement of ionization results from local breaking of oxide bonds and its magnitude is related to the microscopic distribution of oxygen atoms in the target. The Cs enhancement effect is found to be connected with the work function change induced by the Cs on the sample surface, and the ionization process is best described by an electron tunneling model. These differences affect the concentration dependence, alloying effect and the optimum operating conditions in SIMS analysis. © 1986.