A procedure is described for the quantitative estimation of hematite, goethite and lepidocrocite in soil clays by differential X-ray diffraction combined with profile refinement. The method has been applied to clays from 12 palaeosol horizons from south-east England. Samples containing as little as 1per thousand dithionite- extractable iron can be analysed in this way. The hematite:goethite ratio is determined to within ±3per thousand for soil clays having ~ 2.5per thousand of each oxide and to within 10per thousand in less favourable cases. The concentration of oxides in the soil determined solely from X-ray diffraction and from a combined chemical analysis/X-ray method are in good agreement. In addition to improving the accuracy and extending the range of iron-oxide concentrations over which analysis can be carried out, the method may, in principle, be used to provide information about the cell parameters and line profiles of each component present.