Laboratory studies of manganese reduction by naturally occurring reduced inorganic compounds were undertaken, both to study further possiblein situmechanisms of manganese reduction and to examine how manganese redox reactions might be coupled to other biogeochemical processes. Chemical manganese reduction by sulfide (in the presence of excess manganese oxide) was found to be rapid and complete, with all sulfide being oxidized within 5–10 min. The reduction of δMnO2by sulfide involves a two‐electron transfer, with S° the predominant oxidized sulfur product. Using a marine sulfate‐reducing bacterium (Desulfovibriosp.), the kinetics of sulfide‐dependent, bacterially mediated manganese reduction were studied; the rate‐limiting step was bacterial sulfide production. These findings suggest that in stratified marine environments (such as the Black Sea, Saanich Inlet, or certain coastal sediments) manganese reduction should occur just below the oxic‐anoxic (O2/H2S) interface or redox boundary as a result of the chemical reaction between manganese oxides and sulfide produced by sulfate‐reducing bacteria.