Reactions models, based on mass transfer calculations, have been computed for sulfate reduction in closed, anaerobic, marine systems containing (1) inert sediment,(2) CaCO 3, (3) Fe 2O 3, and (4) Fe 2O 3 + CaCO 3. The diagrams resulting from the computations show variations in the concentrations of aqueous species as a function of grams of organic matter oxidized. The calculations show that the buffering capacity increases from system (1) to system (4). The computations also reveal that the asymptotic limit of pH reduction in systems (1) and (2) is 6.60 whereas in systems (3) and (4) it is pH 6.92. The models predict that in system (2) CaCO 3 should dissolve as a result of sulfate reduction whereas in systems (3) and (4) CaCO 3 should precipitate along with iron sulflde. Three natural occurrences of sulfate reduced marine waters are used to show the application of the models. The models are useful in that they illustrate the major factors that control the pH of anoxic waters.