Phase relations in the system iron oxide-manganese oxide in air have been determined by the quenching technique. The system is not binary because of varying oxidation states of Fe as well as Mn. The following crystalline phases occur in the temperature interval studied in the present investigation (800 to 1585 degrees C.): Spinel solid solution (approximately Fe 3 O 4 -Mn 3 O 4 ); hematite solid solution (Fe 2 O 3 with some manganese oxide dissolved in its structure); Mn 2 O 3 solid solution (alpha -Mn 2 O 3 with some iron oxide dissolved in its structure); tetragonal Mn 3 O 4 solid solution (Mn 3 O 4 with some iron oxide dissolved in its structure). There are 2 isobaric invariant situations in the system. At 932 + or - 5 degrees C., Mn 2 O 3 solid solution, spinel solid solution, and tetragonal Mn 3 O 4 solid solution are in equilibrium with gas (pO 2 = 0.21 atm). At 997 + or - 5 degrees C., hematite solid solution, Mn 2 O 3 solid solution, and spinel solid solution coexist in equilibrium with gas (pO 2 =0.21 atm). Melting relations of mixtures in this system are characterized by liquidus and solidus curves going through a temperature minimum of 1565 + or - 5 degrees C. A plot of d-spacing versus composition for the spinel solid solution series (Fe 3 O 4 -Mn 3 O 4 ) shows a pronounced change in slope of the curve at approximately 32 wt.% manganese oxide. In the hematite and Mn 2 O 3 solid solution phases the d-spacings change relatively little with composition, and X-ray methods are not practical as tools for determining compositions of these phases.