WHEN a temperature gradient is applied to a homogeneous solution of an electrolyte, there is a tendency for a concentration gradient to be established (Soret effect). The thermal diffusion potential associated with the temperature gradient should change during the establishment of the Soret equilibrium. For a cell of the type: the change in e.m.f. during the establishment of this equilibrium can be calculated theoretically1–3. In the simplest case2, where the solution contains a uni-univalent salt obeying the ideal law, the change in e.m.f., Δt, for a cell containing electrodes reversible to anions is and, for one with cation-reversible electrodes, where an increase in concentration at the hot electrode is taken as positive, and n + and n − represent the transference numbers of the cation and anion respectively.