The Bi&sngbnd;Ag&sngbnd;O&sngbnd;Cs photocathode has been used for many years because of its high and reasonably uniform sensitivity throughout the visible spectrum. Chemical and physical studies are reported which were made to gain an understanding of the chemical nature and of the energy band model of the material. It was found that the Bi&sngbnd;Ag&sngbnd;O&sngbnd;Cs cathode consists of the semiconductor Cs3Bi, elementary Ag, one or more Cs oxides, and possibly some elementary Cs. The Ag, but not the Bi, can be replaced by several other metals. Energy band models were determined by using considerations which have been successful in the interpretation of the alkali antimonide photoemitters. From optical and photoelectric measurements, values for band‐gap energy and electron affinity were thus obtained for Cs3Bi as well as for the more complex materials represented by the symbols Cs3Bi(O), Cs3Bi(Ag), and Bi&sngbnd;Ag&sngbnd;O&sngbnd;Cs. A band gap of approximately 0.7 ev was found for all of these materials. It was shown that the oxygen reduces the electron affinity of Cs3Bi from 1.3 to 1.0 ev but does not greatly enhance the photoelectric yield. The addition of Ag to Cs3Bi or Cs3Bi(O) greatly increases the yield without appreciably reducing the electron affinity. A hypothesis is suggested to explain these effects.