A numerical model to diagnose the internal conditions of a molten carbonate fuel cell (MCFC) has been developed to calculate both the temperature and performance of stacks. The performance of the stack is evaluated by applying a ＇formula for MCFC performance＇ which has been derived from tests on single small cells with the same active components as the stack. Concerning the separator temperature and the cell performance, calculated results are compared with experimental data acquired during the operation of a 100-kW class stack. Good agreement is obtained. The applied numerical electric circuit model is modified to analyse the voltage distribution within each individual cell. The purpose of the model is to identify the cause of unexpected voltage differences within each cell during operation of a 100-kW class stack. Two causes are identified, namely, increase in the partial internal resistance (IR) and insufficient supply of fuel gas to the cell. The calculated cell voltage distribution and the observed voltage difference for a given cell exhibit similar behaviour. (C) 2000 Elsevier Science S.A. All rights reserved.