Field emission microscopy was used to investigate the thermal desorption of cesium from ZrB2 and TiB2 surfaces. Described is a method of extracting the work function and effective emitting area from the Fowler-Nordheim data with minimum assumptions. Work functions were measured after heating at successively higher desorption temperatures, assuming a value of 2.2 eV for the cesium saturated surface. The minimum value of the work function for the cesium coated ZrB2 surface was 2.04+/-0.1 eV, while that of the TiB2 surface was 1.74+/-0.05 eV. Average work functions of the thermally cleaned ZrB2 and TiB2 surfaces were found to be 3.78 eV and 5.44 eV, respectively. Activation energies for thermal desorption of cesium from ZrB2 and TiB2 were found to be 1.1+/-0.1 and 0.71+/-0.2 eV, respectively.