Diffusion experiments using barium carbonate-alumina couples have been carried out. This work follows up a kinetic study previously published on the synthesis of BaAl2O4 from stoichiometric BaCO3 + gamma-Al2O3 powder mixtures, with formation of an intermediate compound [1]. In the current study, the formation of other barium aluminates is shown when the distances of diffusion greatly increase. After different annealing times, a complete isothermal cross section of the equilibrium phase diagram can be proposed.