The solid-state reaction between barium carbonate and rutile powders to form barium metatitanate BaTiO₃ was studied by thermogravimetric analysis, X rays, and microscopy. Phase-stability domains were drawn in a temperature - P_CO2 diagram. The dependence of the reaction kinetics on P_CO2, P_O2 , or P_N2 is discussed. In particular, the rate continuously decreases when P_CO2 or P_N2 increases, but it reaches a maximum as a function of P_O2. For given partial pressures of gases and temperature the kinetics curves (conversion degree vs.time) may be interpreted by a "hampered"-diffusion mechanism.