Isotopic analyses of dissolved molybdenum are presented for sediment pore waters from a reducing sedimentary basin and for fluids from a low-temperature ridge flank hydrothermal system. δ98/95Mo in these fluids range from 0.8 to 3.5? (relative to a laboratory standard), demonstrating that marine sedimentary reactions significantly fractionate Mo isotopes. Within the upper 3 cm of sediment, manganese oxide dissolution produces an isotopically light fluid relative to seawater (mean of four analyses = 2.1 ¿ 0.1? versus seawater = 2.3 ¿ 0.1?). Below 6 cm depth, authigenic Mo uptake results in an isotopically heavier fluid (up to 3.5?) indicating that reducing sediments are likely to be a net sink for isotopically light dissolved Mo. In contrast, fluid circulation within a low-temperature ridge-flank hydrothermal system is a source of isotopically light Mo to the ocean having an end-member fluid of ~0.8?.