The alkali antimonide compounds are widely applied in light detection devices. The strong optical absorption and the low electron affinity make the photon–electron conversion very efficient in these materials. In this paper, we present the results of a photoemission spectroscopy study of K 3 Sb films of 100Å with and without an extra layer of 0.1ml Cs. The shallow Sb 4d core levels show a chemical shift for K 3 Sb, which is only very slightly affected by the extra Cs layer. The results are explained as a consequence of charge accumulation at the K 3 Sb surface, which is induced by the Cs surface states.