The ternary and quaternary selenides, beta-CsBi3Se5, Rb2CdBi6Se11, CsAg(0.5)Bi(3.5)Se6, CsCdBi3Se6, Rb2Ag(1.5)Bi(7.5)Se13, and Cs2Ag(1.5)Bi(7.5)Se13, are all members of the new homologous series A2[M(5+n)Se(9+n)] and crystallize in structures related to each other in a systematic way; these compounds are mid gap semiconductors and are of interest as thermoelectric materials.