Experimental data for reactions between rhodochrosite and the manganese oxides, supplemented by thermodynamic calculations, indicate that at l000 atmospheres the rhodochrosite field boundary decreases from 715o±5°C,– 12.5 log fo2 (atm.) units on the Mn1-x O-Mn3O1 buffer, to 330°±23oC,–8.4 log fo2 units on the MniO4–MnsO3 buffer, and to 260 ±35o C,–5.6 log fo2, units on the Mn2O3-MnO2 buffer. Rhodochrosite is stable at high temperatures only at low fo2 Comparison of the siderite, calcite, and magnesite stability fields with that of rhodochrosite suggests that the addition of manganese expands the siderite, reduces the calcite, and causes little change in the magnesite fields. Oxygen fugacities encountered in the MnO-C-O system are higher than those in corresponding iron-bearing systems. Metamorphic manganese oxide bodies have high inherited fo2 values due to internal buffering, which causes gradients between such bodies and country rocks containing iron oxides or graphite.