A detailed picture of the behavior of cesium oxide as a low work‐function coating on III‐V semiconductors and on silver has been obtained. Measurement of required cesium and oxygen exposure for optimum photoyield shows that the compound normally formed is close to CS2O, with variations in required exposure for very thin and very thick layers. By making simultaneous Kelvin work‐function, photoyield‐threshold, and thickness measurements, it was possible to establish that the CS2O, ann‐type semiconductor, forms a heterojunction or Schottky barrier with its substrate. This provides a band bending which produces a gradual lowering of the vacuum level with increasing thickness to an ultimate work function of 0.6 eV. The photoyield and dark current from the substrate are limited by the interfacial barrier at the heterojunction. This barrier is 1.00±0.05 eV for a silver substrate and 1.23±0.03 eV for GaSb. The band‐bending distance in the CS2O is about 50 Å and the hot electron scattering distance is 9 Å. These data have been used in an improved calculation of the maximum &Ggr; escape probability and requisite CS2O thickness for electron emission from III‐V semiconductors of different bandgaps. Electron emission from CS2O induced by an oxygen overpressure was also measured. CSOH is compared with CS2O as a work‐function lowering coating.