The preparation of two chromium ores as reference materials is described and the procedures used in the measurement of the homogeneity of the samples are outlined. Statistical analysis of the data submitted by 32 laboratories that resulted in the certification of the samples is presented. Certified values have been assigned for 11 constituents in the one sample and for 10 in the other sample. Provisional values have been assigned for FeO in both samples and CaO in one of the samples.