A Cs liquid metal ion source (LMIS) and focused ion beam system for microarea secondary ion mass spectrometry (SIMS) have been developed. Because of the chemical activity, low surface tension, and high vapor pressure of Cs, few successful Cs LMISs have heretofore been reported, as opposed to the many successful applications of Ga in LMIS. A new Cs‐LMIS design had to be developed to supply Cs to the emitter/reservoir under UHV to prevent oxidation, to prevent liquid Cs from dripping off the emitter substrate, and to minimize the surface area exposed so as to minimize thermal evaporation. The last consideration is very important since the evaporation rate of Cs is many orders of magnitude higher than, e.g., Ga at the melting point. A focused Cs beam was obtained with a newly designed SIMS system, which is about 0.1 &mgr;m beam size and a beam current density on the specimen of 0.8 A/cm2. An operating time of more than 600 h has been achieved at a total emission current of 1 &mgr;A without resupplying the source material at room temperature.