The oxygen permeability, total conductivity and Seebeck coefficient of La2Ni0.9Co0.1O4+δ were studied in the oxygen partial pressure range of 10^{â 16} Pa to 50 kPa at 973â 1223 K. The conductivity of La2Ni0.9Co0.1O4+δ is predominantly p-type electronic within the whole p(O2) range in which the K2NiF4-type structure exists. Thermally-activated mobility, the values of which are 0.02â 0.08 cm² V^{â 1} s^{â 1}, and the p(O2) dependencies of electron-hole transport suggest a small-polaron conduction mechanism. Oxygen permeability of dense La2Ni0.9Co0.1O4+δ membranes, with an apparent activation energy of 192 kJ mol^{â 1} in oxidising conditions, is limited by both bulk ionic conductivity and the surface exchange rate. The role of surface processes in limiting permeation is also significant under air/H2â H2O gradients and increases with decreasing temperature. The stability boundary of the La2Ni0.9Co0.1O4+δ phase at low oxygen pressures is similar to that of undoped lanthanum nickelate, which allows stable operation of nickelate membranes under high oxygen chemical potential gradients, such as air/10% H2â 90% N2, at 973 K. At temperatures above 1000 K, the decomposition products form blocking layers on the membrane surface causing degradation of the membrane performance with time. The average thermal expansion coefficient of La2Ni0.9Co0.1O4+δ ceramics, calculated from dilatometric data in air, is 12.8 à 10^{â 6} K^{â 1} at 400â 1265 K.