The molecular geometry and vibrational frequencies of monomeric and dimeric dysprosium tribromide, DyBr 3 and Dy 2Br 6, together with the electronic structure of their ground and first few excited-state molecules were determined by high-level computations, electron diffraction, gas-phase infrared, and matrix isolation infrared and Raman spectroscopy. The effect of partially filled 4f orbitals and spin-orbit coupling on their structure was studied by computations. While the geometry of the monomer does not depend on the 4f orbital occupation, the bond angles of the dimer are noticeably influenced by it. The monomer is found to be planar from all methods; the suggested equilibrium bond length of the molecule (r e) is 2.591(8) Å, while the thermal average distance (r g) is 2.606(8) Å. Although the gas-phase DyBr 3 molecule is planar, it forms a complex with the matrix molecules in the matrix- isolation spectroscopic experiments, leading to the pyramidalization of the DyBr3 unit. Our model calculations in this regard also explain the often conflicting results of computations and different experiments about the shape of lanthanide trihalides.