La(1 - x)SrxFe(1 - y)GayO3 - delta perovskites are some of the most promising materials for oxygen transport membranes due to their high oxygen semi-permeation and good chemical stability at high temperature under a wide range of oxygen partial pressures (from 021 to 10(-19) atm.). This paper is focused on a better understanding of the influence of the Ga and Sr substitution ratios in the La(1 - x)SrxFe(1 - y)GayO3 (-) (delta) perovskite series on oxygen semi-permeation performances, oxygen diffusion and surface exchange coefficients. This study identifies the optimal compositions of the La(1 - x)SrxFe(1 - y)GayO3 - delta perovskite series. In addition, this study shows that the rate-determining step of oxygen transport through the membrane depends strongly on the Ga and Sr substitution ratios. (C) 2014 Elsevier B.V. All rights reserved.