The chemical diffusivity of 1.8 mol% aluminum-doped Ba-TiO3-delta was measured on single-crystal specimens, as a function of ambient oxygen partial pressure, in the range 10-18 atm less than or equal to P-O2 less than or equal to 1 atm and at temperatures of 800 degrees less than or equal to T less than or equal to 1100 degrees C, via a conductivity-relaxation technique. As in the polycrystalline, undoped BaTiO3-delta described in Part II of this work, the chemical diffusivity exhibited a maximum, of thermodynamic origin, approximately at the stoichiometric composition (delta = 0), The measured diffusivity was analyzed, based on the defect structure proposed and Wagner's classic theory of chemical diffusion, and the mobilities of the electrons and holes, as well as all of the relevant defect-equilibrium constants, then were evaluated with no prior assumptions. The evaluated parameters were compared with those for the undoped BaTiO3-delta.