Gas-phase electron-diffraction (GED) data together with results from ab initio molecular orbital and normal coordinate calculations have been used to determine the structures of the aluminum trihalides AIX(3) (X = Cl, Br. I) and the chloride and bromide dimers Al2Cl6 and Al2Br6. No monomeric species were detected in the vapors of Al2Cl6 at the experimental temperature of 150 degrees C, nor in Al2Br6 at 167 degrees C, but the vapors of AlCl3 at 400 degrees C and AlBr3 at 330 degrees C contained respectively 29 (3)% and 7(4)% dimer and the AlI3 at 300 degrees C about 8% I-2. The known equilibrium symmetry of the dimers is D-2h, but the molecules have a very low-frequency. large-amplitude, ring-puckering mode that lowers the thermal average symmetry to C-2v. The effect of this large-amplitude mode on the interatomic distances was handled by dynamic models of the structures which consisted of a set of pseudoconformers spaced at even intervals along the ring-puckering angle 2 Phi. The ring-puckering potential was assumed to be V(Phi) = V(4)(0)Phi(4) + V(2)(0)Phi(2), and the individual pseudoconformers were given Boltzmann weights. The structures were defined in terms of the geometrically consistent r(alpha) space constraining the differences between corresponding bond distances and bond angles in the different pseudoconformers to values obtained from ab initio calculations at the HF/6-311G(d) level. Results for the principal distances (r(g)/Angstrom), angles (angle alpha,theta/deg), and potential constants (V-i(0)/kcal mot deg(-1)) from the combined GED/ab initio study for Al2Cl6/Al2Br6 with estimated 2 sigma uncertainties are Al-X-b = 2.250(3)/2.433(7), Al-X-t=2.061(2)/2.234(4), XbAlXb = 90.0(8)/91.6(6), X(t)AIX(t) = 122, 1(31)/122. 1(31). [theta]= 180 - 2 Phi, = 165.5(59)/158.2(91), V-4(0) = 0.0/75.0 (assumed), V-2(0) = 25.0/0.0 (assumed). The potential constants could not be refined: although the single-term values listed provide good fits, in each case combinations of quadratic and quartic terms also worked well. For the monomers AlCl3, AlBr3, and AlI3 (D-3h symmetry assumed in r(alpha) space) the distances (r(g)/Angstrom) with estimated 2 sigma uncertainties are Al-Cl = 2.062(3), AI-Br = 2.221(3), and AI-I = 2.359(5) Angstrom. Vibrational force fields were evaluated for all molecules. The experimental, theoretical, and vibrational results are discussed.