Equilibria between species in solution and its corresponding gaseous species were investigated in various molten fluorides mixtures. From the measurements of boron and silicon kinetic constant of gas wrelease, a relative fluoroacidity scale of solvents was proposed. Due to very high kinetic rates of SiF4(g) and BF3(g) release for the most acidic melts, another approach to evaluate fluoroacidity based on mass transport was considered. Relationships between bridging fluorine atoms, viscosity and fluoroacidity was demonstrated. Diffusion coefficients of Si(IV) and B(III) ions were measured and correlated to the viscosity through the Schmidt number (Sc) defined as the ratio between solvent viscosity and solute diffusivity. These results confirmed the fluoroacidity scale previously obtained, validating the original approach developed on mass transport. (C) 2013 Elsevier Ltd. All rights reserved.