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A combined field/laboratory study has been undertaken to explore the mineralogy, concentrations and reactivity (towards sulfide) of iron in marine sediments. Also considered is the importance of bacterial Fe liberation in regulating pore-water chemistry. Two contrasting marine environments are included; one is the relatively Fe-poor FOAM site and the other is the Fe-rich sediments of the subaqueous Mississippi Delta. Results show that oxide minerals are the most important Fe phases in early diagenetic pyrite formation. However, viewed separately, lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite. When Fe oxides are present in relatively high concentrations, dissolved sulfide is nearly absent from sediment pore waters (with concomitant high concentrations of dissolved Fe), even in the presence of active sulfide production by sulfate reduction. A combination of experimental results and diagenetic modelling shows that in some sediments pore water Fe finds it origin in the bacterial reduction of iron oxides. This seems the case even though greater amounts of Fe are reduced by reaction of sulfide with iron oxides. It appears that distinct microenvironments may exist in marine sediments, where, in one microenvironment, sulfide reacts with Fe oxides locally precipitating Fe sulfide minerals. In another, Fe reduced and solubilized by microorganisms migrates freely into solution.