Pyrite textures are described and illustrated and stable S-isotope data are presented from the Alton (Gastrioceras listen) marine horizon of the Westphalian Lower Coal Measures, from sections near Penistone in central northern England, with the object of relating the paragenetic sequence of pyrite formation to the conditions of sediment deposition and diagenesis. The earliest diagenetic pyrite is dispersed as framboidal and related textures. It is followed in the marine shale, coal and ganister by more localised but more intense pyrite deposition and replacement in a variety of textures. Most of this is precompactional in age, but some, together with pyrite in veinlets and cleat, is postcompactional. Marcasite is rare and mainly late. δ34S ratios range between −35·31‰ and +20·39‰. There is a definite trend from lighter values (−1·15 ± 6·47‰) in the marine part of the sequence to much heavier values (+12·73 ± 7·66‰) in the sediment below the coal. This allows the relationship of the earliest pyrite deposition in the coal-peat and ganister to the chemistry of their own depositional fresh water to be seen but then relates the main pyrite deposition to the influx of the marine-water sulphate of the Alton horizon, and shows the penetration of this influence downward into the coal-peat and its seat-bed.