On the basis of the models of various developed high-temperature fuel-cell heat-engine hybrid systems, a unified model of hybrid systems is proposed. General expressions for the power output and efficiency of hybrid systems, high-temperature fuel cells such as solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs), and heat engines including the Brayton, Otto, Diesel, Atkinson, Braysson, and Carnot engines are, respectively, derived by using the theories of electrochemistry and non-equilibrium thermodynamics. The effects of main irreversible losses existing in real fuel cells and heat engines on the performance of hybrid systems are investigated. The general performance characteristics and optimal operating regions of some of the key parameters of hybrid systems are discussed in detail. A variety of special typical cases are discussed. The important results in the literature can be readily reproduced, and the interesting findings of our study are presented. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.