The equilibrium oxygen nonstoichiometry, delta, for the mixed ionic and electronic conducting perovskite La0.9Ca0.1FeO3 - delta (LCF91) was measured for oxygen partial pressures, P-O2, between 10(-3) and 10(-21) atm and temperatures of 750 to 900 degrees C using a tubular, two-probe Pt vertical bar YSZ vertical bar LCF coulometric titration cell. XRD experiments confirmed that LCF91 remains a single phase perovskite under the reducing conditions of a fuel electrode, and reveal no evidence of reaction with the zirconia electrolyte. The P-O2-nonstoichiometry relationship can be interpreted using a dilute-solution point-defect model involving oxygen vacancies, electrons, and electron holes. (C) 2013 Elsevier B.V. All rights reserved.