Abstract Experimental study on the concentration difference cell between seawater and river water (dialytic battery) has been made with special attention to the transient change in the power output. The cell consists of 59 compartments made with 29 ion-exchange membrane pairs, each of which has an effective area of 80 cm 2 per sheet. It has been found that the voltage drop across the load decreases exponentially with two different short and long time constants, of which values are about 50 and 800 s, respectively. Accordingly the power also decreased to about one fourth (from 259 to 68.9 mW/m 2 pair) after sufficient time passed. These transient behaviors have been successfully interpreted by the capacitor-type equivalent circuit model. From comparisons between the experimental and the model results, values of the internal resistance and two kinds of capacitor were determined, of which values were 114 Ω, 1.35 and 66.4 F, respectively. It was suggested that the short and long time constants came from charge double layers formed at the cathode and the anode, respectively. Measurements of potential difference between saline water compartments in the case with and without external current by reference electrodes showed that voltage losses occurred due to the concentration polarization near the ion-exchange membranes and the activation polarization at the electrode surfaces.