The molecular structures of the monomers and dimers of all iron trihalides, FeF3, FeCl3, FeBr3, and Fel3, were determined by computations, and the structure of iron trichloride monomers and dimers was determined also by gasphase electron diffraction. The thermal-average bond length, rg, of FeCl3 is 2.136(5) Å, and the estimated equilibrium bond length from experiment, 2.122(6) Å, is in excellent agreement with the computed value. Vibrational and thermodynamic properties have also been calculated for all species. The stability of iron trihalides decreases toward the iodide; neither the bromide nor the iodide can be evaporated without decomposition. The structures of isomers of trimers and tetramers for iron trichloride have also been calculated. The connection between the gas-phase and crystal-phase structures and the exceptional stability of the dimer is discussed.