The mineralogy of seven weathered profiles derived from Triassic shales in the Sydney area has been examined by X-ray and chemical techniques. The illite of the parent shales is converted, through the loss of potassium ions, to montmorillonite-illite inter-layered minerals in the leached shale and mottled zones; but in the soil zone, where maximum leaching occurs, these inter-layered minerals tend to form kaolinite and a 14Å mineral with properties similar to vermiculite. It is considered that the oxidation of adsorbed ferrous ions has been responsible for the development of the vermiculite-like mineral. Though quartz is lost from the profiles, the kaolinite content increases surfacewards at the expense of illite and related degraded products.