Partial reduction reactions were used to form a metallic phase either around or inside oxide grains in polycrystals in the Fe-Mn-O system. By suitable choice of oxide composition, partial pressure of oxygen. annealing time and temperature, it is possible to control the nucleation and growth of the metallic phase to produce a wide range of metal-ceramic microstructures. These include ceramic grains with a thin layer of metallic phase at their boundaries; ceramic grains with a thick layer of metallic phase at their boundaries -essentially, a metal ceramic composite; and ceramic grains containing a fine distribution of metal particles-essentially, a ductile phase toughened ceramic. The presence of the metallic phase increases the fracture toughness of all the metal-ceramic microstructures with respect to that of the pure ceramic, with the largest increase observed for the metal ceramic composite. It is believed that the principles established by studying the Fe-Mn-O system can be used on more practical mixed oxide systems to produce metal ceramic microstructures, which in some cases are unique, and in particular, if the starting oxide material is in the form of plate- or rod-shaped crystals, to produce metal ceramic composites in situ.