Mixed ionic–electronic conducting La0:5Sr0:5 Fe1 xBxO3 δ (B: Al, Cr, Zr, Ga, Ti, Sn, Ta, V, Mg, and In with x=0, 0.1, 0.2) perovskite materials were produced via solid-state synthesis. In order to study the effect of Bsite substitution on the expansion behavior of these materials, their thermal expansion in air up to 900°C and isothermal expansion at the same temperature from air to Ar were measured by dilatometry. Ti and Ta were found to be the most effective substitutions in suppressing the isothermal expansion. The isothermal expansion at 900°C from air to Ar was reduced by 50% by substitution of 20% Ti or 10% Ta. Therefore, these compositions were further characterized by 4-probe total DC conductivity and permeation measurements under air/Ar gradient. The total conductivity of La0:5Sr0:5FeO3 δ was decreased by more than one order of magnitude at low temperatures and from 430 S/cm, which is the maximum, to around 100 S/cm at 500°C with the addition of Ti and Ta. The normalized oxygen permeation of LSF at 900°C decreased from 0.18 to 0.05 μmol/cm2s and 0.07 μmol/cm2s with the substitution of 20% Ti and 10% Ta, respectively.