This work illuminated characteristics of the inert gas step addition (ISA) method with various time ranges of reactant-flow shift in a 100 cm(2) class molten carbonate fuel cell. The flow shift caused by the volume between the addition port for the inert gas and the cell was controlled by the length of 0.6 cm tubes in a range from 0.8 to 4.8 m at the anode as well as the cathode. Anodic voltage peaks provoked by the inert gas addition depended on the tube length and became blunt at longer tube lengths, indicating that the anodic overpotential is a function of reactant flow rate. The anodic overpotential estimated by the ISA depended on the tube length, but it became a constant at longer tubes. The cathode showed a flat peak at the moment of inert gas addition regardless of the tube length, indicating that the cathode has a relatively small gas-phase mass-transfer resistance. The cathodic overpotential with ISA showed an insignificant dependence on tube length. (C) 2004 Elsevier B.V. All rights reserved.