Solubility values and mass transfer data for the Ni2+ and Co2+ ions are important for modelling studies of the dissolution of the state-of-the-art NiO as well as the alternative LiCoO2, MCFC cathode material. Unlike the spectroscopic techniques used predominantly in dissolution studies, electrochemistry allows for the determination of mass transfer data beside solubility values. These quantities were studied using an electrochemical technique following a specific measurement procedure, named "the addition method" here. The diffusion constants of Ni2+ and Co2+ ions in molten carbonate at 650 degrees C were obtained, 9 10(-6) and 2 10(-6) cm(2)/s respectively. Lower solubility values for both ions in mixtures with increasing lithium content were obtained under 100% CO2. Combined activation energies for the product c*root D in saturation were obtained of 10 kJ/mole for CoO and 13 kJ/mole for NiO. Under standard cathode gas the NiO solubility in the standard 62/38 Li/K melt was determined at 0.35 mu mole/cm(3), in the alternative melt composition 60/40 Li/Na, the solubility was below the detection limit <0.05 mu mole/cm(3). In both melts a LiCoO2 solubility of 0.2 mu mole/cm(3) was obtained.