Multicollection system allows improvement of measurement reproducibility for isotope ratios on SIMS instruments provided that the other sources of measurement variability are well controlled. This paper presents analytical protocols developed to control the mass fractionation in the collection optical system and the variation of electron multiplier yield. Results for both silicon and oxygen isotope ratios demonstrate that an experimental error as low 0.2 per mil is achievable whenever both isotopes were measured either with Faraday cup detectors or combining one Faraday cup and one electron multiplier (EM). (C) 2004 Elsevier B.V. All rights reserved.