The variations of the clay minerals illite and chlorite were measured by XRD in the <2 ¿m fraction of four loess sections along a North-South transect on the Chinese Loess Plateau (CLP). The ratio of illite to chlorite (I/C) provides a tool to evaluate strong monsoon conditions on the CLP. I/C ratios above 2.5 indicate strong chlorite (silicate) weathering, a process that takes place under a regime which, at present, in the southern CLP has annual precipitation and temperature exceeding ~600 mm and 10¿C, respectively. The weathering regime during the last previous interglacial period in the southern CLP may have been warmer and had higher rainfall. During weathering the chlorite may release iron, so that nanometer-sized iron oxide minerals may form, and the free Fe2O3 and magnetic susceptibility are increased in the resulting paleosol. If that has occurred in the four sections studied, it is only in the southern two.