The continental slope off southwestern Taiwan is characterized by rapid sedimentation as a result of high denudation rates in southern Taiwan. Analyses of total organic carbon (TOC), total sulfur (TS), total iron (FeT) and cold acid extractable iron (FeA) in sediment samples from piston and box cores show unusually low sulfur contents (0.01–0.12%) in sediments against a background of moderate organic carbon contents (0.5–1.3%) and rich reactive iron (1.2–2.0%). Consequently, the degrees of sulfidation (less than 0.1) are very low for iron. Porewater sulfate concentration dropped rapidly in the top 30 cm of the piston core but decreased very gradually below. Below the top layer of relatively rapid sulfate depletion, only 5 mM of porewater sulfate was further consumed as the depth reaching 3 m below the sediment surface. The low content of reduced sulfur and low porewater sulfate depletion in the sediment column suggest very slow net sulfate reduction, which may result from competition for organic matter degradation or re-oxidation of reduced sulfur during microbial reduction of Fe(III) or Mn(IV) oxides. This may explain the exceptionally low S/C ratios (~ 1/30) and low degrees of sulfidation (mostly below 0.06) found in sediments off southwestern Taiwan. In light of the high fraction of fossil carbon in river-borne sediments observed in northern Taiwan, the highly refractory terrigenous material, which probably constitutes a major fraction of sedimentary organic matter in the study area, may also limit sulfate reduction.