N 2 adsorption at 77 K, temperature-programmed desorption (TPD), X-ray and neutron diffraction, and high-resolution electron microscopy (HREM) were used to characterize a Ce 0.8 Tb 0.2 O 2-x /La 2 O 3 -Al 2 O 3 oxide. This research shows that the supported cerium/terbium mixed oxide has good thermal stability up to 700 °C under H 2 (5%)/Ar and above this temperature in air or inert atmosphere. A combination of HREM and hydrogen chemisorption is shown to be a good method for estimating the dispersion of the supported cerium/terbium mixed oxide. The parallel study of its structural evolution also provides some interesting data on the deactivation of the supported Ce 0.8 Tb 0.2 O 2-x by reaction with the supporting alumina at 900 °C under H 2 (5%)/Ar and the formation of a LnAlO 3 perovskite phase.