The significance of thermal diffusion in liquid systems with forced convection has been investigated over a wide range of temperature. Numerical calculations have been carried out for the case of dilute liquid solutions that flow into the entrance region of a tube or an annulus, and the theory has been developed for a first order chemical or electrochemical reaction that takes place on the electrode surface. As an example, dilute aqueous electrolyte solutions at sub-critical temperatures (up to 350 degrees C) have been considered. As the Lewis number and the Soret coefficient increase, the impact of thermal diffusion on mass transport is predicted to become increasingly important at higher temperatures. Simple analytical relations for describing thermal diffusion effects in liquid solutions subject to forced convection have been derived, and good agreement between these analytical solutions and numerical results is shown. The use of an electrochemical cell with forced convection could be a promising experimental technique for determining Soret coefficients for electrolyte solutions and liquid mixtures at elevated temperature.