The chemical diffusivity of 'undoped' polycrystalline BaTiO3-delta was determined via a conductivity relaxation technique, at elevated temperatures (800 less than or equal to T/degrees C less than or equal to 1100) as a function of the ambient oxygen partial pressure in the range of 10(-16) less than or equal to P-O2/atm less than or equal to 1 including an n- to p-type transition regime. Mathematical formulation was developed to convert conductivity relaxation to the corresponding nonstoichiometry (delta) relaxation in the transition regime. It has been found that the chemical diffusivity appears to exhibit a maximum at the n-to-p transition point where the electronic minimum conductivity falls, and that surface reaction becomes more rate-determining than diffusion as the transition point is approached from both n-type and p-type branches. Experimental details are given and the results are exhaustively compared with those reported up to date on the 'undoped' BaTiO3.