Summary: The behaviour of iron in the weathering of biotite in a sedentary soil profile developed on appinite has been investigated using the Msbauer effect. Both Fe²⁺ and Fe³⁺ in sites with cis and trans hydroxyl groups were observed at various stages in the weathering sequence. The results indicate that there is little or no ordering of the octahedral cations in the fresh biotite and hydrobiotite, but in the more weathered interstratified vermiculite-chlorite samples Fe³⁺ concentrates in the more distorted sites. No evidence was obtained for the presence of an anisotropic recoil-free fraction.