We report the (bare) surface redox-reaction rate constant (k) over tilde that was determined, along with the chemical diffasivity (D) over tilde, by a conductivity relaxation technique on Al-doped single crystal and undoped polycrystal BaTiO3-delta as a function of oxygen activity in its range of - 16 less than or equal to log a(O2) less than or equal to 0 at elevated temperatures of 800-1100 degreesC. It takes a value in the range of - 4 < log((k) over tilde /cm s(-1)) less than or equal to - 1, which is even larger than that of the oxides that are considered best as oxygen membranes. It has been found that the surface reaction step grows more rate controlling as the electronic transference number gets smaller or the electronic stoichiometric composition (delta approximate to 0) is approached. The oxygen potential drop due to the surface reaction was estimated by an oxygen concentration cell technique. The oxygen potential drop grows larger as the stoichiometric composition is approached, that is in accord with the variation of (k) over tilde against oxygen activity. (C) 2003 Elsevier Science B.V. All rights reserved.