The heterojunction model of a highly efficient, semitransparent, multialkali-antimonide Na2KSb(Cs,Sb) photocathode. consisting of a homogeneous, polycrystalline, p-type Na2KSb base layer and an n-type CsxSb surface film (0 < x less than or equal to 3), has been investigated, The thin CsxSb film was deposited onto the Na2KSb base layer by alternating introductions of Cs vapour and evaporations of Sb at elevated temperatures in an ultrahigh vacuum established in the standard image intensifier tube. An estimation of this film’s thickness, which amounts to between 2.5 and 8.5 nm for the stoichiometric ratio 1 less than or equal to x less than or equal to 3, was obtained using a hypothetical growth model of a thin film barrier one cluster-size thick on the photosensitive base layer surface while analysing the photocurrent behaviour both Juring the introductions of Cs vapour and during the evaporations of Sb. This model was based on the growth of regular pyramids with a square net base of 4 x 4 atoms and equilateral triangle sides, on the apparent surfacer and on a quantum mechanics scattering cross-section for elastic scattering of photoelectrons on the base boundary atoms. Comparisons of our calculated CsxSb-Cs dissociation vapour pressure relations with the reported P-Cs(T) expressions for Cs description from the Na2KSb(Cs) photocothodes and of our estimated Sb MNN Auger signal asymmetry of the CsxSb compounds with the reported Sb MNN line shape of the highly efficient Na2KSb(Cs) photocathodes were made, both leading to x = 1.5. The thermodynamic considerations of the synthesis reactions existing in the Sb-Cs binary system which gave their Cs equilibrium pressures were performed using thermodynamic functions related to real solid solutions. The Sb MNN asymmetry of the CsxSb compounds was obtained by means of a thermodynamically estimated valence charge transfer in CsxSb, supposing their mutual linear dependence.