The Clusius-Dickel (CD) technique couples thermal diffusion to a counter-current, natural convective flow to effect separation of molecules of different molecular weights or shapes in liquid or gaseous solutions. Materials separable by the technique include: aqueous biological solutions, isotopes in both gas and liquid states, aqueous solutions of both ionized and un-ionized materials, organic solutions, liquid metallic solutions, fused salt solutions, and gas mixtures including ortho and para-hydrogen.