Abstract: Goethite (Gt) and hematite (Hm) are important iron minerals in soils and sediments. However, conventional measurements can not effectively distinguish them in the low concentrations typical of soils and sediments. In this paper we demonstrate that diffuse reflectance spectroscopy (DRS) in conjunction with thermal treatment provides a definitive technique for identifying and estimating concentrations of Gt and Hm in soils and sediments. The DRS pattern of Gt exhibits diagnostic changes of the first derivative value (FDV) peaks after heating to 300℃. Hm can be identified by its FDV peak without heating because it displays no shift in peak wavelength after heating. The degree of thermal change induced in a sample depends on the concentrations of both Gt and Hm. Application of this technique to the samples of the Tertiary red clay from Loess Plateau indicates the concentrations of about 0.3~0.4% for Hm and 0.5~1% for Gt.