Thermionic emission measurements have been used to determine the work function (&Jgr;) of pure and oxidized uranium samples between 1100 and 1300 K; Auger electron spectroscopy (AES) was used to verify the cleanliness and compositions of the samples. It was found that impurities present in ppm amounts in the bulk U segregated to the surface upon heating and had an appreciable effect on the zero‐field emission currents as well as the slopes of the Schottky curves obtained at various temperatures. A combination of ion‐sputtering and ultrahigh vacuum (UHV) annealing at high temperatures was successful in reducing the total impurity level on the hot surfaces to 5%. At this low concentration of impurities, well‐behaved Richardson line plots were obtained withA=135 A cm−2 K−2and &Jgr;=3.54 eV for pure U andA=128 A cm−2 K−2and &Jgr;=3.19 eV for UO2. The Schottky coefficients for clean U approached their ideal values at fields >400 V/cm.