The startup procedure can have a significant impact on the performance and long-term stability of a SOFC. It is often bound up with a modification in electrode microstructure and electrode/electrolyte interface chemistry during first electrical loading of a membrane electrode assembly (MEA), resulting in a significant decrease of the Area Specific Resistance (ASR). This effect, termed cell activation or cathode formation process, was investigated for electrolyte supported MEAs and linked to solid state diffusion processes at the interface cathode/ electrolyte. In this paper, a system and control theory based model is presented which simulates the cathode formation process taking place during first electrical loading and dynamic SOFC stack operation. The model comprehends linguistic expert knowledge and provides information about internal cell variables and the responses to changes of operating conditions. This interdisciplinary approach allows a prediction of the time dependency of stack performance during startup procedure as well as during dynamic load cycles. (C) 2004 Elsevier B.V. All rights reserved.