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Clay mineralogy of the Jiujiang soils, classified as red earth (on summit and slope), were investigated using X–ray diffraction (XRD), cation exchange capacity (CEC) determination, and high-resolution transmission electron microscopy (HRTEM). The XRD results show that the soils included kaolinite, illite, vermiculite, and mixed-layer I/S and K/S as clay minerals. In XRD patterns of clay separates the low-angle shoulder on the kaolinite (001) peak shifted to ~ 8 A on glycolation, indicating the presence of mixed-layer K/S clays in the soils. Kaolinite content in the interstratified kaolinite–smectites was estimated to be ~ 95% for samples of the middle to lower soil profile and ~ 60% for that of the upper portion, respectively. A broad reflection for 10–14 A disappeared and an 18.7 A peak occurred in the XRD patterns after formamide treatment. The 10 A peak became much more intense on heating to 400 °C, indicating that the 10–14 A peak was expandable and easily contractible and thus originated from mixed-layer illite–smectite (I/S) clays. After heating to 550 °C, the 7 A peak disappeared while the 10 A peak became more intense and showed a low-angle tail, indicating the interstratified smectite component was hydroxy interlayered. The higher CEC values of the Jiujiang clay separates (22.2–27.3 cmol/kg) also pointed to the occurrence of smectite in the soils. HRTEM analysis showed different lattice fringes for clay particles in the samples; a great majority of layer spacings were in the range 10–20 A, with a minority of ~ 7 and ~ 25 A. Particles with ~ 7 A fringes showed a lateral transition of two 7 A kaolinite layers merging into one 12 A smectite layer. This can be interpreted as the partial intercalation of one smectite layer in kaolinite. There was no long-range order in the stacking sequence of K/S layers, and the interstratification was of a random nature. In the uniform soil horizon of the upper profile, there was more vermiculite and illite, but less I/S and K/S, while in the net-like soil horizon of the middle-lower profile there was more kaolinite, I/S, and K/S, with only a trace of vermiculite. The occurrence of K/S clays confirmed that the Jiujiang red earth was derived from syndepositional pedogenesis and the climates changed from warm and seasonally dry to warm and more humid conditions during the soil-forming period.