A-site deficient (Pr0.9La0.1)1.9Ni0.74Cu0.21Ga0.05O4+δ ((PL)1.9NCG), with the K2NiF4 structure, is found to exhibit higher oxygen transport rates compared with its cation-stoichiometric parent phase. A stable oxygen permeation flux of 4.6 × 10−7 mol cm−2 s−1 at 900 °C at a membrane thickness of 0.6 mm is measured, using either helium or pure CO2 as sweep gas at a flow rate of 30 mL min−1. The oxygen flux is more than two times higher than that observed through A-site stoichiometric (PL)2.0NCG membranes operated under similar conditions. The high oxygen transport rates found for (PL)1.9NCG are attributed to highly mobile oxygen vacancies, compensating A-site deficiency. The high stability against carbonation gives (PL)1.9NCG potential for use, e.g., as a membrane in oxy-fuel combustion processes with CO2 capture.