Residual soils are weathering products of rocks that are commonly found under unsaturated conditions. The properties of residual soils are a function of the degree of weathering. A series of index properties, engineering properties, mercury porosimetry tests and scanning electron microscope (SEM) examinations were performed on residual soils from two major geological formations in Singapore. The results indicate that the variation in the index and engineering properties as well as microstructural characteristics of the residual soils with depth can be related to the degree of weathering. An increase in the degree of weathering results in an increase in pore volume and produces a larger range of pore-size distribution. The variation in the pore volume and the pore-size distribution through a profile of weathered rock can be used as an indicative measure of the variation in the degree of weathering with depth.