Due to relatively high oxygen permeability, lanthanum-strontium ferrite phases are of interest as ceramic membrane materials for the partial oxidation of natural gas, This work was focused on the study of perovskite-type ferrites co-doped with Sr2+ and Ce4+ or Nb5+, with particular emphasis on the ionic transport and thermodynamic stability limits at low oxygen chemical potentials. Dense membranes of La0.5-2xCexSr0.5+xFeO3-delta (x = 0 - 0.2) and La0.5-2ySr0.5+2yFe1-yNbyO3-delta (y = 0 - 0.1) were characterized employing X-ray diffraction (XRD), scanning electron microscopy (SEM), dilatometry, oxygen permeation and faradaic efficiency studies, and the measurements of total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10(-20) to 0.5 atm. The incorporation of Ce4+ or Nb5+ was found to decrease thermal expansion and electronic transport parameters, whereas the ionic conductivity behavior is complex, indicating the relevance of redox interactions of the variable-valence cations and the concentration of mobile oxygen vacancies.