O2- concentration relaxation experiments in a 52/48 moi % Li/Na molten carbonate melt saturated with NiO were performed. The Ni2+ was added to act as a probe for the O2- concentration. The dynamic behavior of the CO2 and O2- concentration during the relaxation of the melt back to the initial equilibrium was studied by recording square wave voltammograms. Our experiments show clearly that CO2 dissolution is the slowest step in the relaxation. Consequently the values for the recombination reaction rate obtained in other studies are not correct, since in them a fast CO2 dissolution was assumed.(1,2) A CO2 gas dissolution rate of 25(+/-5) 10(-4) cm/s and Henry's constant for CO2 of 20(+/-3) mu mol atm(-1) cm(-3) at 650 degrees C could be determined. Our numbers were used in simple calculations of the performance of the porous molten carbonate fuel cell cathode, and indicate that the CO2 dissolution rate determines the lower limit for p(CO2) Of 0.05 arm at 150 mA/cm(2) for proper cathode operation.