Weathering of Oxford Clay has been studied in five profiles from different areas. The well-known weathering reaction (pyrite + calcite→gypsum) is the major compositional change. The calcite distribution in vertical profiles is distorted by the introduction of calcite-rich solifluction material above in situ weathered clay. Although the correlation of calcite with depth is low, that of calcite with undrained shear strength is high. It is concluded that, although interparticle bond weakening by calcite solution is occurring, the calcite introduced from overlying material may be acting as a cement. Variations in bulk clay mineralogy are small and do not follow consistent patterns. However, the crystallinity of illite in the clay fraction decreases towards the surface; this may reflect the degradation of the crystal structure at the particle margin, where bonding occurs. The weakening of overconsolidated clays is interpreted in terms of bond weakening following oxidation of pyrite; most weathering changes are concentrated near the base of the weathered zone, where this reaction occurs.