Energy balance measurements of evapotranspiration from a developing corn crop are compared with daily equilibrium evapotranspiration estimates to examine the accuracy of the model and the environmental conditions under which it can be applied. Equilibrium estimates compared closely (a standard error of 6%) with the measured values when the surface was moderately dry, a condition which applied to 14 of the 24 days of the experiment. The ratio of actual evapotranspiration to available energy and the Bowen ratio are used to establish moisture and temperature limits for the model. The success of the model was related to a typical diurnal pattern of the difference between actual and equilibrium evapotranspiration which reflects expected variations of moisture stress during daytime hours. The performance of the model was nearly independent of the physical condition of the surface and the height of the required air temperature measurement. An equation is presented which permits easy calculation of equilibrium evapotranspiration from air temperature, net radiation, and soil heat flux data.