In the paper, the development of an expansive residual clay soil profile weathering from a Permian mudstone is described. The characteristics considered include soil mineralogy, soil chemistry, soil texture and engineering parameters including expansive potential. The effects of weathering are shown to be most evident in the development of a series of texturally distinct soil horizons. It is shown that the chemical and mineralogical characteristics of the parent mudstone are very similar to those of its derived expansive clay soils, even though the expansive potential in the clay soil is almost twice as great as in the parent mudstone. The origins of an enhanced expansive potential in the B horizon are explored, and it is found that, for the weathering of a mudstone in a temperate climate, the effects of weathering on structure are more important than the effects of weathering on mineralogy. It is concluded that most of the smectite clays in the weathered clay soils were inherited from the parent mudstone (with little or no transformation) and that these clay components were likely to have been present in the sediments from which the mudstone formed. The enhanced expansive potential in the residual clays is attributed to the physical effects of weathering in breaking down structure and/or bonding in the parent rock, which reduces the potential expansiveness of the clays contained in the parent rock. This proposal is supported by experimental evidence.