The determination of chemical diffusivity by a conductivity relaxation technique requires a quantitative correlation between thr conductivity relaxation (<(sigma)over bar>(t)) and the corresponding nonstoichiometry relaxation (<(sigma)over bar>(t)), Unlike in an exclusively n- or p-type regime, the quantitative correlation is by no means straightforward in a mixed n/p regime. A mathematical formulation for the correlation between the local conductivity (sigma(x,t)) and nonstoichiometry (sigma(x,t)) is developed, and the correlation between their spatial averages <(sigma)over bar>(t) and<(delta)over bar>(t) examined via computer simulation for totally diffusion-controlled and surface-reaction-controlled cases. It has been found that, for the latter, the minimum of <(sigma)over bar>(t), <(sigma)over bar>(m), equals that of the equilibrium conductivity, sigma(m), corresponding to the n-to-p transition point, and otherwise <(sigma)over bar>(m) > <(sigma)over bar>(m). This fact can thus be used as a criterion to determine the exclusively surface-reaction-controlled kinetics. The results are confirmed experimentally through the analysis of conductivity relaxation across the n-to-p transition point of 1.8 m/o Al-doped BaTiO3-delta.