Work‐function changes during the desorption of cesium from three tungsten surfaces, polycrystalline foil, (100), and (110) oriented single crystals, have been measured in the region of the work function minimum. A modification of the vibrating capacitor technique which made contact‐potential difference on electron emitting surfaces possible was developed for these experiments. For different arrival rates of cesium, the minimum work function occurred at different temperatures and decreased in magnitude with temperature. Values at 295°K were 1.78 eV for polycrystalline foil, 1.795 eV for the (100), and 1.955 eV for the (110). For all specimens the decrease in magnitude with temperature was of the order of 10−3eV·K0−1. These data were compared with thermionic values for the work function minimum and with theoretical predictions. We note especially that the sign of the variation is opposite to that predicted by the Rasor‐Warner theory in every case.