The alpha activity of soils and the degree of the equilibrium of the thorium series has been related to the age of soils developed on a truncated laterite landscape in southwestern Australia. The uplift of the old lateritic plateau has formed a sequence of erosional and depositional surfaces which form the parent materials of the present-day soils. These surfaces because of their different relative ages have been subjected to different degrees of weathering and leaching. The alpha activity of the soils formed on these different landscape surfaces is influenced firstly by the amount of weathering that the surface has undergone, and secondly by the degree of leaching that the soil has undergone as evidenced by profile development. It has been found that the younger soils have higher alpha activities with the thorium series tending more towards equilibrium when compared with older soils, where the alpha activity is lower due to the leaching of the daughter nuclides from the profile.