We report on Knudsen effusion mass spectroscopic experiments, aimed at the detection of intermetallic molecules in the vapor over mixtures of antimony or bismuth with thallium or elements of groups 14 and 16 and of binary molecules containing elements of groups 14 and 16. The discussion is concentrated on the following species : PbSb2Te, Pb2Te2, SnSbBiTe, SnBi2Te, Sn2Te2, and TlSbTe2. All these molecules are present in the respective vapors with comparatively high abundances. They contain four atoms and 20 valence electrons. Furthermore, the 22-valence-electron species Sb2Te2 and Bi2Te2 can be observed. The tetraatomic species are isoelectronic to Sb-4 or Sb-4(2-). Hence it can be assumed that the 20-electron species have tetrahedral structures, while the 22-electron species are planar.