Low-temperature fluid flow has been identified as an important component of the heat, mass and chemical flux at sites of active venting. One way to study the movement of water through hydrothermal deposits is to analyse the pore-fluids extracted from sulfidic sediments. The composition of diffuse flow fluids in one sediment core from the south of the active TAG mound reflects the dilution of high-temperature, black smoker fluids, with modification inside the mound during ongoing circulation. Fluid modification includes conductive cooling, removal of Si from solution and entrainment and removal of U from seawater which can represent significant sinks of these elements. Precipitation of clay minerals and Fe-oxides is currently occurring within this sediment core generating ochreous type material subsurface. Extensive low-temperature alteration of basaltic material at the core-top is apparent in conjunction with supergene enrichment of Cu and Mn. ¿ American Geophysical Union 1996