Pyrochemical separation processes are considered to treat spent nuclear fuel and particularly to separate fission products from actinides. In order to estimate the efficiency and selectivity for various extraction processes based on a molten salt/solvent metal separation technique, we have to know the properties of the elements to be extracted in each solvent, notably their activity coefficients in the two phases. The classical way to measure the activity coefficient of an element in a liquid metal is to use a concentration cell whose the electromotive force is measured. This type of cell involves two electrodes: (a) the element investigated in its pure metallic form and (b) the element solvated in the solvent metal. The electrolyte used for this study is a chloride melt that contains the element under consideration as a solute. In this paper, an effort was made to measure activity coefficients in liquid metals by means of electrochemical techniques rather than a potentiometric technique. The experimental protocol was optimized by measuring the activity coefficient of gadolinium in liquid gallium (solvent metal) (Gd/Ga) at 530°C for several amounts of gadolinium in gallium, and log (Gd/Ga) was determined to be equal to -10.17 (mole fraction scale). Then, the temperature dependence of the activity coefficient was determined in the range of 535 to 630°C. It appears that log (Gd/Ga) varies linearly with the reciprocal value of T, thus following the theoretical variation. The electrochemical method was also performed to determine the activity coefficient of plutonium in liquid gallium at 560°C. The value of log (Pu/Ga) so obtained is equal to -8.04 (mole fraction scale). This value was confirmed using electrochemical and potentiometric measurements with a plutonium-saturated gallium electrode.