Secondary ion mass spectrometry (SIMS) measurements of δ18 O in nine chromite (Mg, Fe)(Al, Cr, Fe) 2O 4 samples (Cr # =0.58–0.79, Mg # = 0.39–0.72) from chromitites and harzburgite have confirmed their intra- and inter-grain isotopichomogeneity, making them suitable reference materials for in situ analysis. The SIMS measurements of the oxygen isotopiccomposition of these chromites exhibit a range of instrumental mass fractionation (IMF) as a function of the end-membercompositions of the chromite, particularly the magnetite (FeOFe 2O 3 ) and spinel (MgAl 2O 4 ) components. Functionalrelationships have been developed to correct for IMF over the compositional range X Mag = 0.01 to 0.05, and X Spl = 0.14to 0.22. We conclude that, to be accurate, SIMS measurements of δ18 O must be combined with electron probemicroanalyses of the individual analysed spots