This paper addresses the off-design and transient response of a high efficiency hybrid system based on the coupling of a recuperated micro-gas turbine (mGT) with a tubular solid oxide fuel cell (SOFC) reactor. The work focuses on the anodic side where an ejector exploits the pressure energy of the fuel to recirculate part of the exhaust gas, in order to maintain a proper value for the steam-to-carbon ratio (STCR) and to support the reforming reactions. Two different stand-alone time-dependent ejector models are presented and validated against experimental data. Then, the most suitable model for cycle simulations, in term of calculation time, has been employed for the transient analysis of the entire hybrid system. The SOFC hybrid system transient behaviour is presented and discussed at several operating conditions from an electrochemical, fluid dynamic and thermal point of view. (c) 2005 Elsevier B.V. All rights reserved.