[en] The compounds of the composition BaLn₂(MoO₄)₄ have been synthesized and their properties described (Ln=La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu and Y). It has been shown by infrared spectroscopy, X-ray diffraction and thermography that the synthesized compounds crystallize in three different structure types. Molybdates of Pr, Nd, Sm, Eu, Gd, Dy, and Lu refer to the first type; the second type comprises the molybdates of Ho, Er, Tm, Yb, or Y; the third type includes only one compound BaLa₂(MoO₄)₄. The existence of the compounds of the composition BaLn₂(MoO₄)₄ has been proved experimentally for the whole lanthanide series and Y. The relation between the properties and structure of the compounds and the nature of the lanthanide has been shown. It has been demonstrated that in each structure type the properties of the compounds are in a functional dependence from the atomic number of r.e.e