This study is mainly concerned with sulfur isotope (delta S-34) values of inorganic sulfur species associated with the North Thrace Basin's coals (Saray and Pinarhisar coalfields). Sulfur isotope analyses were performed on pyritic sulfur, native sulfur, sulfate sulfur (rozenite) and the precipitated sulfate (P-BaSO4) (leached sulfate from the coal). The delta S-34 values of pyritic sulfur, rozenite, native sulfur and P-BaSO4 samples in Saray coals ranging from -6.9 parts per thousand to -37.3 parts per thousand are comparable to the delta S-34 values of Pinarhisar coals which change from -1.1 parts per thousand to - 59.3 parts per thousand. A wide range of strong negative delta S-34 values in both coalfields, particularly in Pinarhisar site, indicate that bacterial sulfate reduction (BSR) and subsequent pyrite formation were the key processes in the peat depositional environments. As in the delta S-34 values of both coalfield sites, comparable Sr/Ba and Th/U ratios and pyrite forms (cleat filling and partly massive) of the coals imply that besides marine sulfate hydrothermal solutions, originated from volcanism being active during peat accumulation in the basin, also contributed to the sulfur pools in the coal depositional environments.