We discuss thermally driven transport in both organic solvents and electrolyte solutions and, in particular, the variation of the thermophoretic mobility D(T) with temperature. We find that the T-dependence in organic solvents arises from the viscosity eta(T), in qualitative agreement with data on polystyrene in ethylbenzene. A more subtle effect occurs for charged colloids in an electrolyte, where the strong correlation of D(T) with the thermal expansivity beta of water is traced back to the thermoelectric effect. Our results provide an explanation for experiments on various colloidal suspensions and, in particular, for the observed change of sign of D(T) as a function of temperature.