The current density on the emittiner surface of a Zr/O/W(100) Schottky electron source has been determined for temperatures in the range of 1600-1850 K and fields in the range of 5 X 10(8)-8 X 10(8) V/m. The work function at different temperatures can be found by fitting the theoretical equation for current density in the extended Schottky emission regime to the experimental current density with the work function as fitting parameter. A linear relationship between work function and temperature was found with a positive coefficient of 0.6 meV/K. This temperature dependence causes the reduced brightness of Zr/O/W(100) Schottky electron sources to be much less dependent on temperature than previously assumed. This implies that these sources can be operated at lower temperatures without losing very much in brightness, improving the lifetime due to a better tip shape stability. (c) 2006 American Vacuum Society.