Thermionic emission measurements on polycrystalline hafnium in the temperature range 1250 to 1820°K have yielded a work function of 3.60 ev and a thermionic constantAof 22.9 amp cm−2deg−2. Intermittent heating thereafter for a total of 46 hours at temperatures above 2100°K and at pressures below 1×10−9mm Hg did not change these parameters appreciably. No gas was desorbed from the hafnium when it was flashed to 2150°K after having been cold for 64 hours. A second sample of hafnium was used as a target in apparatus for studying electron ejection by ions. It was more brittle than the first and thus more contaminated with gas. Its work function was found to be 3.91 ev and the constantAto be 20.5 amp cm−2deg−2after repeated flashing to 2150°K. Evidence from the electron ejection work indicates that the surface of this second hafnium sample was covered with an appreciable fraction of a monolayer immediately after cooling from high temperature. One can conclude that the work function of hafnium increases as the metal irreversibly absorbs gas on heating.