Sulfur transformations in freshwater peat were studied using contrasting stable isotope signatures of atmospheric input (high δ³⁴S) and Sphagnum peat substrate (low δ³⁴S). Wet subsurface peat samples from the Lehstenbach watershed, Fichtelgebirge, Germany were incubated anaerobically at 5 and 15°C. Pore-water sulfate was augmented with natural precipitation at the onset of the experiments. Sulfate concentrations and δ³⁴S ratios of residual pore water were measured in 1-day intervals (9 days) and 1-week intervals (7 weeks) at 15°C, and in 1-week intervals (7 weeks) at 5°C. Initially, SO₄²⁻ concentrations decreased (by 50 to 85%) and δ³⁴S ratios increased (by as much as 16‰) at both temperatures due to bacterially-mediated dissimilatory sulfate reduction. At the higher temperature (15°C), the S isotope effect (Δ δ³⁴S) was higher than at the lower temperature (5°C). On day 4 (at 15°C) and day 29 (at 5°C), the δ³⁴S ratio of pore-water sulfate started to decrease by as much as 20‰. The changing S isotope composition provided evidence for a dynamic turnover of the pore-water sulfate pool in anaerobic peat. The observed δ³⁴S pattern could not be explained solely by isotope selectivity of the sulfate-reducing bacteria. Sulfur isotope data indicated a replenishment of the sulfate pool by hydrolysis of ester-sulfate.