A miscibility gap exists in the quasi‐binary solid solution Co2TiO4 – CoAl2O4 as verified by experimental investigations and theoretical calculations. The critical temperature is ca. 1700 K. Spinodal decomposition was observed in this two‐cation‐sublattice system, if the spinel solid solution had been undercooled into the region of the coherent spinodal at T = 973 K. Simultaneous ordering occurs in certain regions of the crystal. The growth‐rate of the spinodal wavelength λ varies linearly in time. This unusual quasi‐ripening process is discussed in relation to the characteristic features of diffusion in (semiconducting) oxides.

Dedicated to Professor Dr. E. Wicke on occasion of his 80th birthday Decomposition morphologies of supersaturated spinel solid solutions in the quasi-binary system Co2Ti04-CoAl20 4 at 973 K show all pertinent features of spinodal decomposition. Since the de composition morphology may not be considered as sufficient evidence for a spinodal process, the thermodynamics of the spinel solid solutions and the linear transport theory of spinodal decompo sition were combined to corroborate the experimental results of the early decomposition reaction.Simultaneous ordering, a common property of ternary and higher nonequilibrium solid solutions, was also observed. These ordering processes occur locally in distinct regions of the spinel crystal. Moreover, the spinodal wavelength X increases in time according to X cc t, which is unusual and will be discussed in the light of cation diffusion in semiconducting oxides with two cation sublattices.