The oxygen non-stoichiometry was determined by coulometric titration for the perovskite oxides La0.2Sr0.8FeO3-delta and La0.2Sr0.8Fe0 8B0.2O3-delta (B = Ti4+ and Ta5+) in the temperature range 600 degrees C <= T <= 900 degrees C and the oxygen partial pressure range: 1.10(-15) < po(2) < 0.209 atm. The non-stoichiometry (delta) is observed to decrease with B-site substitution of Fe. The data can be well fitted with simple defect chemistry models. At low oxygen non-stoichiometry all compositions show a deviation from a localized electrons defect model. The standard and partial molar thermodynamic quantities were obtained and a gradual transition from localized to itinerant electrons with decreasing non-stoichiometry is proposed from the delta-dependency of the configurational entropy. The absolute value of the enthalpy of oxidation decreases upon B-site substitution of Fe proposing a decreased thermodynamic stability for the substituted materials. The electrical conductivity was measured at T = 900 degrees C in the oxygen partial pressure range: 1 . 10(-17) <= po(2) <= 0.209 atm. The electrical conductivity and charge carrier mobility decrease upon 20% substitution of Fe roughly by a factor of 2, but do not show a significant dependence on the nature of the B-site dopant. (C) 2013 The Electrochemical Society. All rights reserved.