The electrical conductivity of La0.2Sr0.8Fe0.55Ti0.45O3-delta (LSFTO) was measured in the temperature range 750 <= T <= 1000 degrees C and at oxygen partial pressures of 1x10(-18)<= p(O2)<= 0.3 atm. The conductivity was measured on both increasing and decreasing p(O2) to ensure that equilibrium was reached. Long equilibration times were required at intermediate oxygen partial pressures between 10(-4) and 10(-11) atm. The pressure dependence of the conductivity in this p(O2) range differs from that previously reported for related compositions. The mobilities of the charge carriers were derived from the stoichiometry and conductivity data. The average hole and electron mobilities of LSFTO at 800 <= T <= 1000 degrees C are 0.015 and 0.0017 cm(2) V-1 s(-1), respectively. The oxygen nonstoichiometry (delta) as a function of p(O2) was determined by solid-state coulometric titration. Data were obtained in the temperature range, 750 <= T <= 1040 degrees C, and in the p(O2) range, 10(-17)<= p(O2)<= 0.3 atm. A point defect model was used to fit the dependence of delta on p(O2). The partial molar thermodynamic quantities for oxidation were calculated and the oxygen nonstoichiometry data were related to the electrical conductivity data. (c) 2005 The Electrochemical Society. All rights reserved.