Detailed studies of the chemical composition of interstitial waters in the sediments obtained along a drill hole transect across the accretionary prism of the Northern Barbados Ridge have revealed a complex set of processes: (1) In Plio-Pleistocene volcanic ash-bearing sediments increased concentrations of dissolved calcium and decreases in dissolved magnesium are the result of volcanic ash alteration; (2) below the decollement large concentration increases in dissolved calcium, accompanied by large decreases in sodium as well as a granual decrease in dissolved magnesium, suggest exchange of these constituents with the deeper seated rocks of layer 2 of the oceanic crust; (3) low chloride concentrations, particularly associated with the decollement zone separating the converging plates and underthrusted sandstones, and also with recent faults in the accretionary complex, suggest advective transport of low chlorinity waters from deeper within the accretionary complex. In and below the decollement zone elevated concentrations of methane of thermogenic origin have been measured.