AbstractSamples in the form of metallic disc are frequently used in the analysis of cast iron by x‐ray emission spectrometry and give good results for common analyses (determination of carbon, sulphur, phosphorus, silicon, manganese, copper, etc.) because of the uniform distribution of the analyte in the sample. However, the results of the analysis of metallic disc samples of cast iron to which cerium had been added were poor, which could be attributed to a non‐uniform distribution of compounds which cerium formed with sulphur and oxygen. A more detailed investigation of cerium distribution in relation to its content in the sample gave evidence of the presumed non‐uniform distribution. Scanning electron microscopy confirmed the presence of unequally distributed aggregates of different size containing cerium, sulphur and oxygen. On the basis of an examination and comparison of published pretreatment procedures, a modified procedure including cerium precipitation with oxalate without transformation into the oxide form is proposed. Using this method, greater reliability of the results was obtained than in the analysis of cast iron samples without pretreatment, i.e. in the disc