The need to develop mathematical models that are as simple as possible but yet encompass all major features of the underlying physical and chemical phenomena in a system under study is a central issue in engineering science and technology. In the macro and meso-scale modeling of solid oxide fuel cells there is a need to develop accurate expressions that relate the composition in the fuel and air channels with the composition in the three phase boundary where all important electrochemical reactions are taking place. In this work the available models are reviewed and some new results are presented and discussed. (C) 2011 Elsevier Ltd. All rights reserved.