Regional “background” δ 34S values of pyrite-(marcasite) nodules throughout the Zechstein basin in Poland have been measured to help estimate the proportion of externally derived sulphur in the Kupferschiefer Cu-Ag ores. The δ 34S values of the 17 FeS2 nodules measured range widely, from -25.2 to -51.9%., similar to the previously published -28 to -43%. range in disseminated pyrite in the Kupferschiefer. The wide variation cannot be attributed to pyrite versus marcasite mineralogy, amount of contained chalcopyrite or sphalerite, carbonate versus shale host rock, early versus late formation, percent of included calcite, or to size, shape, or texture. There is also no relation with proximity to the centres of copper mineralization in southwestern Poland where sulphides are typically isotopically heavier. The δ 34S values do, however, vary directly with percent of host-rock fragments included in the nodules. Repeat samples that were washed with acid or hot water show the same wide variation, indicating that contamination by sulphate sulphur in the host rock is not a factor. Neither is organic sulphur because of its small volume. Instead, the sulphur composition may be fundamentally controlled by the formation mechanism of the nodule, whereby 34S-rich sulphide is preferentially concentrated, possibly replacing anhydrite lenses. Alternatively, a network of host rock inclusions might act as a more accessible conduit for later, 34S-rich fluids to infiltrate the nodule and add to earlier, 34S-poor pyrite. In the ore deposits, higher δ 34S values of ore nodules suggest less indigenous sulphur in limestone than shale lithologies. An isotopic temperature of 61 °C from a chalcopyrite-galena pair agrees with other estimates of <105°C. Higher values in ore nodules/veinlets than in adjacent disseminations, and the calculated δ 34Spy value from a pyrite-bornite mixture support the idea that metal-bearing 34S-rich fluids penetrated the Kupfer-schiefer through a network of fractures.