The development and use of the chemostat, an apparatus which controls pH, Eh and temperature during redox reactions, is described. Its application is illustrated for the synthesis of the birnessite variety of manganese dioxide, via 2(x-1)MnO4- + (7-2xMn2+ + (8-3x)H-2 <-> 5MnO(x) + 2(8-3xH+ where x is the oxygen index of the manganese dioxide. The chemostat controls the pH and Eh of the precipitation reaction by the judicious addition of KOH and KMnO4 solutions, respectively, as well as maintaining a constant temperature in the precipitation cell. Using the control conditions outlined herein, the average errors associated with the pH, Eh and temperature were +/- 0.2 pH units, +/- 0.025 V and +/- 0.5 degrees C, respectively. The birnessite samples that were produced were characterized in terms of X-ray diffraction, x in MnOx and open circuit voltage measurements. These properties were found to have a strong dependence on the synthetic conditions.