In electrolyte solutions, the differential migration of the ionic species induced by the presence of a thermal gradient leads to the buildup la steady-state electric field. Similarly to what happens for the Seebeck effect in solids, the sample behaves therefore as a thermocell. Here, we provide clear evidence for the presence of thermoelectric fields in liquids by detecting and quantifying their strong effects on colloid thermophoresis. Specifically, by contrasting the effects of the addition of NaCI or NaOH on the Soret effect of micellar solutions of sodium dodecyl sulfate, we show that the presence of highly thermally responsive ions such as OH- may easily lead to the reversal of particle motion. Our experimental results can he quantitatively explained by a simple model that takes into account interparticle interactions and explicitly includes the micellar electrophoretic transport driven by such a thermally generated electric field. The chance of carefully controlling colloid thermophoresis by tuning the solvent electrolyte composition may prove to be very useful in microfluidic applications and field-flow fractionation methods.