V. Summary 1. This article describes the comparative results of the determination of shrinkage constants (Haines’ method) and of certain soil moisture constants for a series of clay soils representative of three important classes, namely, tropical red soils derived from igneous rocks and from limestone, kaolinitic soils and materials, and grey and yellow soils , mainly derived from aqueous sedimentary rocks. 2. An attempt is made to interpret the data in terms of Wilsdon's micro-reticular hypothesis of soil colloidal structure, and to explain the physical significance of some of the constants measured, particularly sticky point moisture ( S ), loss on ignition ( I ), water uptake from aqueous vapour ( H, R ), and Haines’ “residual shrinkage” moisture content ( d ). 3. The results indicate that tropical red soils closely resemble kaolinitic materials in their shrinkage behaviour and moisture relations , but that they are intermediate between this type and grey and yellow soils, which exhibit characteristic high total shrinkage, marked residual shrinkage and high adsorptive capacity for moisture and for cations. 4. A brief review of recent literature concerning the mineralogy of clay minerals (based on X-ray analysis and optical examination) is offered in support of these findings. 5. The need of detailed mineralogical investigations of the products of weathering of rocks under humid tropical conditions is urged. Such study might reveal the causes of the characteristic and peculiar field properties of tropical red soils which appear to have been neglected by soil physical-chemists.