Kinetic and thermodynamic studies of the terbium oxides between the Tb 2O 3+δ ( n = 4) and Tb 7O 12 ( n = 7) phases of the homolgous series, Tb n O 2 n−2 , have been carried out as a function of oxygen pressure at 708, 740, 772, and 806°C. The thermodynamic study shows a reproducible hysteresis loop which depends on the temperature and pressure. The kinetic study was carried out by measuring the weight gain of an oxide sample in an oxygen atmosphere as a function of time. The results are interpreted by a mixed-model mechanism of diffusion and phase boundary reaction control. The relative fit of the data with other models, including the phenomenological model previously found satisfactory for a related reaction, is shown. The kinetics in the final stage are strongly affected by the existence of pseudophase behavior near Tb 7O 12. From measurement of the oxygen pressure and temperature dependence of the observed rate constants, the activation energies for the diffusion process and phase boundary reaction process were determined to be − 14 and 10 kcal/mole, respectively.