A current-interrupter method has been employed to measure the potential relaxation from output to open-circuit voltage (OCV). The relaxation process represents the voltage losses of molten-carbonate fuel cells which are composed of ohmic loss (eta(IR)), reaction overpotential (eta(Re)), and Nernst loss (Delta E-loss). The responses of current interruption for a 100 cm(2) class Li-Na carbonate fuel cell were measured as functions of temperature, gas utilization, and oxidant gas composition (O-2/CO2). The single cell showed three different relaxation patterns of time regions during potential decay to OCV; the shortest time region (less than 20 mu s) is due to eta(IR), an intermittent time region (20 mu s to 150 ms) is due to eta(Re), and the residual time region is due to Delta E-loss. The further analysis of eta(Re) data is consistent with the argument that the oxygen reduction reaction in the single cell is controlled by a mixed diffusion process of superoxide ion (O-2(-)) and CO2.