Values for the work functions of thermionic emitter surfaces composed of alkaline earth oxides have been approximated by means of a closed energy balance cycle which is a modification of the Born-Haber Method ordinarily used to estimate lattice energies or electron affinities. Comparison of calculated surface potentials with reported work functions for alkaline earth oxides disclosed reasonable agreement. The modified Born-Haber cycle includes, among other terms, the lattice energy, the heat of sublimation, and the molecular electron affinity of the gaseous, neutral, diatomic molecules under consideration. Justification for the incorporation of these quantities into the energy balance, together with the general implications of this method of estimating work functions, is discussed.