In the first part of this paper we present a detailed model of an oxygen mixed conducting membrane (OMCM) monolith for air separation. In addition to the oxygen separation, the OMCM operates as a heat exchanger at elevated temperature and pressure. The model is based on energy and species conservation balances. It is shown that the oxygen permeation through perovskite-related materials is strongly dependent on the oxygen partial pressure difference across the membrane and on the temperature of the solid wall. The numerical results obtained from steady state as well as transient simulations agree well with the available data. In the second part, linear model reduction is applied to the OMCM model. The method for model reduction used here, balanced residualisation, reduces states while preserving steady-state behaviour. The comparison of two reduced-order models (19 states and 5 states, respectively) with the full-order model (>2000 states), reveals good agreement for frequencies lower than 1 Hz. The reduced models simulate faster, and can be used for controllability analysis and control design. (C) 2009 Elsevier B.V. All rights reserved