Soil kaolins are unlike reference mineral kaolins. They commonly have high-defect structures, complex crystal morphologies, very small crystal size, and appreciable structural iron. In many tropical and sub-tropical soils, kaolin is the dominant clay mineral and strongly influences many surface chemical reactions. Current X-ray diffraction (XRD) ‘crystallinity indices’ cannot distinguish between quite different soil kaolins so alternative criteria are required. The possibility that kaolin properties are related to their susceptibility to expansion by the hydrazine–water–glycerol (HWG) intercalation procedure was investigated for soil kaolins from Indonesia, Australia, and Thailand, and some reference kaolins. The soil kaolins contained minor impurities, mostly quartz, anatase, illite, and hydroxy-interlayered vermiculite. Percentages of different types of kaolin (Types A, B, C and D, in order of increasing defect structure) were calculated from the height of the XRD reflections at about 1.0 and 0.7 nm after intercalation treatment. Results showed that the different percentages of the various types of kaolin were related to cation exchange capacity, surface area and other properties of the kaolins. Although all samples contained a mixture of kaolin types, the soil kaolins had larger proportions of high-defect kaolin Types C and D; most reference clays were dominated by low-defect Types A and B. Particle morphology determined by electron microscopy did not enable recognition of all high-defect kaolins in either reference or soil kaolins. Statistical analysis revealed that the soil and reference kaolins clustered into distinct groupings on the basis of their properties and the proportions of different kaolin types. The main discriminating factors were particle shape (notably percentage tubes + spheres) and degree of crystal ‘disorder’ as indicated by the proportions of the different kaolin types. The HWG procedure has the potential to discriminate between high and low-defect kaolins and to differentiate kaolins even when impurities are present.