Pyrite is a ubiquitous mineral in shales. It results from the bacterial reduction of seawater sulphate to sulphide and the reaction of that sulphide with iron to form iron sulphides and subsequently pyrite. Five Jurassic shales were examined. 1 The Upper Oxford Clay (Oxfordian) Warboys Cambridgeshire. 2 The Dunans Clay (Callovian-oxfordian) Isle of Skye. 3 The Lower Oxford Clay (Callovian) East midlands. 4 The Posidonia Shales (Lias ε) Southern Germany. 5 The Dunans Shale (Callovian) Isle of Skye. The five shale units are divisible into three major biofacies "Normal" (Units 1& 2), "Restricted" (3) and "Bituminous" (4 & 5), which show varying pyrite contents and isotopic compositions, that reflect the changing influence of the three limiting factors of pyrite formation (sulphate, organic carbon, and iron). Samples of shale were analysed for fine grained pyrite content and isotopic composition, organic and carbonate carbon, and hydrochloric acid soluble iron. The Normal shales showed low pyrite sulphur (<1.3%) and organic carbon (<2%), degree of pyritization (D. O. P. ) (<0.5) and negative (<-28%o) δ34S values. These are consistent with the formation of pyrite near the sediment surface, limited by organic carbon. In the Restricted shales higher pyrite sulphur (av. 1.4%) and organic carbon (av. 5%), degree of pyritization (0.5 to 0.7) and less negative (>-28%o, <-9%o) δ34S values indicate the continuation of pyrite formation deeper into the sediment, where it is limited by sulphate diffusion. High D. O. Ps in the Posidonia Shales (>0.8) suggest that pyrite formation is limited by the availability of iron in Bituminous shales. Heavier δ34S values (>-28%o) in the Posidonia Shale suggest limitation of sulphate reduction by sulphate diffusion, and lighter values in the Dunans Shale (<-28%o) suggest limitation by organic matter. Petrographic and isotopic studies of shell replacement and associated concretionary pyrite from the Lower Oxford Clay show types of replacement related to the differences between the porewaters within the shell and those of the shale, particularly as regards iron supply.