An electron beam scanning technique has been devised to measure variations in the surface work function. These measurements indicate a wide patch distribution in the range of 3.25 to 4.5 eV for inactive uranium carbide, which upon thermal activation becomes covered with a surface having a work function in the narrow band of 3.0 to 3.25 eV. The active surface is very sensitive to ambient gases and can be easily poisoned to a value of work function higher than was the case for the inactive state. These results support the hypothesis that a uranium film is responsible for the activation as was suggested by previous thermionic measurements. An analysis of the effect of a nonuniformity in work function on thermionic measurements, shows that experimental Richardson plots or effective work function plots can give highly erroneous results. The magnitude of this error can be determined and is expressed in terms of the spread of the patch distribution.