Surface water, pore water, vegetation and peat cores were sampled from a waterlogged ombrotrophic peat bog on the Shetland Isles, UK and analysed for different S forms and their isotopic composition, in an attempt to elucidate the biogeochemical processes affecting S during peat diagenesis. Surface waters show that inputs of S to the peat have a maritime-dominated isotopic composition close to +20‰ CDT. Uptake of S by vegetation introduces a −10‰ shift in δ 34S from these input values. Below the vegetation layer and down to 18 cm depth, bacterial SO 4 2 - reduction is the major control on S species distribution and isotopic composition within the solid peat and pore waters. In this part of the peat, preferential reduction of 32SO 4 in pore water during metabolism produces isotopically light sulphide, which is incorporated into the solid phase in both inorganic and organic forms, while pore water SO 4 2 - becomes enriched in 34S. From 18 to 28 cm, organic S content falls relative to C and residual organic S becomes 34S-enriched, indicative of mineralization of organic S, a process which releases isotopically light S to the pore waters. Still deeper in the core (28 to ∼50 cm), bacterial reduction of pore-water SO 4 2 - , now enriched in 34S, results in addition of isotopically heavy S to the solid phase. Limited pore water data suggest that below 50 cm mineralization reactions again release S from the organic fraction of the peat.