To control the collector temperature of an electrostatic probe, we have studied and built a cooling circuit to cool by convection; we have considered two possibilities for the cooling liquid—with and without phase conversion. We give the corresponding thermic study and the cooling effects on the experimental results, obtained from a simple Langmuir probe and a grid probe. The cooling is indispensable at all temperatures for a probe with a positively biased grid. We find that the reemission current varies when the collector is still negative.