The contact potential difference in a low‐pressure thermionic energy converter can be written asCPD=[open phi]˜E−[open phi]˜C, where[open phi]˜Eand[open phi]˜Care averaged work functions of the emitter and collector, respectively. The estimation of[open phi]˜Eand[open phi]˜Cfrom existing work‐function data is discussed. As an example,[open phi]˜Evalues are estimated for well‐aged polycrystalline tungsten wire and those estimates are compared with experimental values.