The vibrational spectrum of lanthanide molybdates (materials with important optical, electrical and catalytic properties) was assigned by using sites symmetry and factor group analysis. Raman and infrared spectra were registered and the modes corresponding to the MoO42- and LnO(n) polyhedra were specified. An interesting behaviour for the symmetric stretching mode of the molybdate ion was found in these condensed lattices. The lanthanide cation does not influence the MoO42- vibrational behaviour. The use of this spectroscopy to characterize these materials is thus shown.