Electrochemical impedance spectroscopy measurements made on bench-scale molten carbonate fuel cells have been used to obtain more information on the inside state of a cell than polarization measurements alone offer. In the frequency range 1 kHz to 10 mHz, the impedance response exhibited two semicircular arcs before and after approx. 3 Hz. An equivalent circuit model and a reactor model were employed to enhance the understanding of the nature of the low-frequency arc (3 Hz- 10 mHz) in the impedance response at nearly open-circuit voltage. The size of the low-frequency arc, which was obtained by the measurements, increases as the flow rate of the gas fed into the cell decreases. The diameter of the low-frequency arc and the fuel flow rate are in inverse proportion to each other. This relation provides useful information for estimating the fuel gas distribution among the stack's cells. (C) 2003 The Electrochemical Society.