The formation mechanism and microstructural development of the spinel phases in the Co 1−x O/Co 2 TiO 4 composites upon reactive sintering the Co 1−x O and TiO 2 powders (9:1 molar ratio) at 1450°C and during subsequent cooling in air were studied by X-ray diffraction and analytical electron microscopy. The Co 2 TiO 4 spinel occurred as inter- and intragranular particles in the matrix of Ti-doped Co 1−x O grains with a rock salt-type structure during reactive sintering. The submicron sized Co 2 TiO 4 particles were able to detach from grain boundaries in order to reach an energetically favorable parallel orientation with respect to the host Co 1−x O grains via a Brownian-type rotation/coalescence process. Upon cooling in air, secondary Co 2 TiO 4 nanoparticles were precipitated and the Ti-doped Co 1−x O host was partially oxidized as Co 3−δ O 4 spinel by rapid diffusion along the {111} and {100}-decorated interphase interface and the free surface of the composites.