The oxygen partial pressure (pO(2))-dependent structural behaviors of two dense tubular ceramic membranes in composition SrFe0.2Co0.8Ox with cubic perovskite structure have been investigated by high-temperature neutron powder diffraction: one in "static" mode and one in simulated-operation mode in which one side of the membrane was exposed to air and the other side to reducing gases with variable pO(2) levels. Rietveld analysis on data collected for the membrane without pO(2) gradients showed that the perovskite is stable in pO(2) down to ∼ 10(-12) atm, and at ∼ 10(-14) atm it starts to decompose into a three-phase mixture containing layered intergrowth Ruddlesden-Popper phases Srn+1(Fe,Co)(n)O-x with n = 2 and 3, along with CoO with rocksalt structure. Similar phase evolution was observed when insufficient air flowed on the air side of the membrane exposed to a pO(2) gradient. The data support a nonlinear model of oxygen content in perovskite across the membrane thickness, corresponding to a pO(2) profile that is shallow inside and steep near the reducing side surface. Gas compositions measured with mass spectrometry indicated that oxygen is permeated from the air side to the reducing side of the membrane. The oxygen permeation fluxes at 900° C were estimated to be 0.4-0.9 sccm/cm(2) for the ∼ 1 mm thick membrane containing perovskite, depending upon pO(2) gradient.