The effect of operational potential on the cell voltage decay rate in a phosphoric acid fuel cell was estimated using the calculation of the platinum dissolution rate. The voltage loss, due to deterioration in activation polarization, was used in calculating the cell voltage decay rate. The voltage loss due to activation polarization was estimated using the relationship between the activation polarization and the platinum surface area. The change in the Pt surface area, arising from the dissolution of platinum, was obtained under accelerated open circuit voltage (o.c.v.) conditions. The study was focused on typical voltage decay rates from 0.1 to 10mV per 1000 h. It was found that the cell voltage decay became significant even at low temperature during long-term operation (at o.c.v.) and that the cell had to be operated below 840 mV (iR-free) at 200 mA cm(-2), 200 degrees C for a decay rate of I mV per 1000 h. From the present estimation, operational conditions such as temperature, cathode potential, and holding time at a given potential, can be roughly determined for a given decay rate. It was concluded that the voltage loss due to platinum dissolution may be negligible at a rated power operation.