Pore-water sulphate concentrations show marked increases at depths >50 m at ODP sites 888 and 890/889 from the Cascadia Margin accretionary wedge. In the uppermost 10 m sulphate concentrations decrease with depth and sulphate delta (super 34) S and delta (super 18) O increase as sulphate is removed by bacterial sulphate reduction. Isotopic data show that sulphate formed below 50 m results from oxidation of early diagenetic pyrite and that oxygen in the sulphate molecules is derived from pore water. Fe (super 3+) in the sediment is the probable oxidizing agent. The increased sulphate concentrations stimulate bacterial sulphate reduction at depths of 70-250 m and are thus important in sustaining deep bacterial activity.