Transport numbers were determined in the pure fused salts, LiNO₃, NaNO₃, KNO₃, and AgNO₃ by measuring the volume changes of the material in the electrode compartments which took place during electrolysis of the melt. Transport numbers in the two binary fused salt systems AgNO₃-NaNO₃ and AgNO₃- KNO₃ were determined by measuring the volume changes accompanying electrolysis and the concentration changes accompanying electrolysis.