Addition of five types of organic matter to Lake Washington sediments resulted in release of high concentrations of iron, organic carbon, and manganese into the interstitial water, and caused an increase in observed sediment oxygen consumption rates. The depressed electrode potentials (Eh< —150 mV) that should accompany such reduction processes did not occur, indicating thatEhwas being poised by redox systems present in the sediment. Iron redox systems [Fe(OH)3‐Fe2+, Fe3(OH)8‐Fe2+, and Fe(OH)3‐Fe3(OH)8] were shown to be poising theEhof control sediments throughout 13 weeks of incubation and dominating the potential of several of the organically amended sediments following the first three weeks of incubation. Depression of calculated iron systemEovalues relative to that of the control sediment early in the incubation appeared to be due to the decreased pH and non‐equilibrium conditions in the organic matter‐amended sediment during the first weeks of incubation. Manganese redox systems exerted no discernable impact on theEhof the sediment.