Different models of the steady-state ionic and electronic defect distributions in mixed ionic-electronic conductors under chemical and electrical potential gradients are compared in this paper. Simple analytical expressions without any dummy parameters are obtained under the local charge neutrality approximation and constant electrical field approximations, respectively. The accuracies of the approximate solutions are evaluated by numerically solving the Nernst-Plank-Poisson equation set using the Galerkin-weighted residual finite element method. The numerical results show that the constant electrical field approximation introduces errors of less than 10% so long as the average ionic transference number is not smaller than 0.5; and that the local charge neutrality approximation introduces only very small errors (less than 3%) even when the average ionic transference number is as small as 0.001. Analytical criteria for the constant electrical field approximations to be valid are also given, and the effects of dopant concentration and the sample thickness are discussed. The analytical criteria are in agreement with the numerical results. (C) 2004 The Electrochemical Society.