The mechanisms and quantitative models for how oxygen is separated from air using ion transport membranes (ITMs) are not well understood, largely due to the experimental complexity for determining surface exchange reactions at extreme temperatures ( > 800 degrees C). This is especially true when fuels are present at the permeate surface. For both inert and reactive (fuels) operations, solid-state oxygen surface vacancies (6) are ultimately responsible for driving the oxygen flux,J(O2). In the inert case, the value of delta at either surface is a function of the local P-O2 and temperature, whilst the magnitude of delta dictates both the J(O2) and the inherent stability of the material. In this study values of delta are presented based on experimental measurements under inert (CO2) sweep: using a permeation flux model and local PO2 measurements, collected by means of a local gas-sampling probe in our large-scale reactor, we can determine directly. The ITM assessed was La0.9Ca0.1FeO3-delta (LCF); the relative resistances to K-O2 were quantified using the pre-defined permeation flux model and local P-O2 values. Across a temperature range from 825 degrees C to 1056 degrees C, delta was found to vary from 0.007 to 0.029 (<1%), safely within material stability limits, whilst the permeate surface exchange resistance dominates. An inert J(O2) limit was identified owing to a maximum sweep surface delta, delta(inert)(max). The physical presence of delta(inert)(max) is attributed to a rate limiting step shift from desorption to associative electron transfer steps on the sweep surface as P-O2 is reduced. Permeate surface exchange limitations under non-reactive conditions suggest that reactive (fuel) operation is necessary to accelerate surface chemistry for future work, to reduce flux resistance and push delta past delta(inert)(max) in a stable manner. (C) 2015 Elsevier B.V. All rights reserved.