In this work equations are presented to describe solute concentration profiles in thermal field-flow fractionation that account for the effect of the temperature drop across the solute zone on the thermal and ordinary diffusion coefficients. The influence of this effect, together with the effects of the temperature dependence of the solvent viscosity and solvent thermal conductivity, on the conversion of retention data into thermal diffusion data were studied. The systematic error made when the ratio of the thermal and ordinary diffusion coefficient (alpha/T) is assumed to be constant can be considerable and is larger for systems with low retention (e.g., for low molecular weight or small thermal diffusion). For the two systems studied (polystyrene in THF and in ethylbenzene), it was found that the temperature dependence of the solvent viscosity is of much greater importance than the temperature dependence of the solvent thermal conductivity. When all three parameters are considered to be independent of the temperature, the results are still quite acceptable. This is due to the fact that the effect of the temperature dependence of the solvent viscosity is counteracted by the combined effects of the temperature dependence of the solvent thermal conductivity and of alpha/T.