The chemical diffusion coefficient of BaTiO3-delta was determined by a conductivity relaxation technique as a function of oxygen partial pressure in the range 10(-15) less than or equal to P-O2/atm less than or equal to 1, including an n-to-p-type transition regime, at elevated temperatures of 1073 less than or equal to T/K less than or equal to 1373. The chemical diffusivity varies convex-upwardly in the range 10(-5) less than or equal to (D) over tilde /cm(2) s(-1) < 10(-2) at all temperatures and its maximum falls approximately at the oxygen partial pressure where the n-to-p transition occurs. This behavior can be explained by the ambipolar diffusion of mobile oxide ions and electrons. From the isotherms of total conductivity and chemical diffusivity as measured, we evaluated the defect parameters including the mobilities of electrons and holes. (C) 2000 Elsevier Science B.V. All rights reserved.