The defect chemical relationship in Nd doped BaCeO3 was discussed based on the defect concentration determined by thermogravimetry and electrical conductivity measured by D.C. 4-terminal method as a function of oxygen partial pressure, P(O-2), water vapor partial pressure. P(H2O), and temperature. In BaCe0.9Nd0.1O3-delta, the defect concentration strongly depended on P(O-2), P(H2O) and temperature. This is different from the results of other acceptor-doped BaCeO3 such as BaCe0.9 Ma(0.1)O(3-delta) (M = Y and Yb), which the concentration of protonic defects is independent of P(O-2). The electrical conductivity of BaCe0.9Nd0.1O3-delta, showed unique P(O-2) dependence at high P(O-2) atmospheres compared to other acceptor-doped BaCeO3. At temperature above 973 K, sigma increased with increasing P(O-2). However, at temperature below 873 K, sigma decreased with increasing P(O-2). This is also different from the general conductivity scheme in other acceptor-doped BaCeO3 that the sigma values increase with increasing P(O-2) in high P(O-2) atmospheres. A different behavior of Nd doped BaCeO3 was explained by considering the changes of oxygen vacancy concentration accompanied by the valence changes of Nd ions between trivalent and tetravalent states. (C) 2010 Elsevier B.V. All rights reserved.