The energy distribution of electrons emitted from a ZrO/W electron source with a radius of curvature of 0.9 μm was determined for extraction voltages ranging from 3000 to 6000 V and emitter temperatures from 1200 to 1900 K. Full width at half maximum values of the energy spectra between 0.3 and 0.8 eV were found. The experimental data are analyzed according to the extended Schottky model for electron emission, yielding the electric field at the emitter surface. The tunneling probabilities are evaluated numerically by integrating the one-dimensional Schrodinger equation, and analytically by employing the commonly used Wentzel–Kramers–Brillouin approximation. Both approaches give good agreement with the experimental data, except for small differences probably due to Coulomb interactions (Boersch effect). The same analysis is also applied to the experimental results for a Schottky emitter with a radius of curvature of 0.3 μm, taken from literature.