Gas-phase electron diffraction (GED) data together with results from ab initio molecular orbital calculations (HF and MP2/6-311+G(d,p)) have been used to determine the structure of hexamethyldigermane ((CH3)(3)Ge-Ge(CH3)(3)). The equilibrium symmetry is D-3d, but the molecule has a very low-frequency, large-amplitude, torsional mode (phi CGeGeC) that lowers the thermal average symmetry. The effect of this large-amplitude mode on the interatomic distances was described by a dynamic model which consisted of a set of pseudoconformers spaced at even intervals. The amount of each pseudoconformer was obtained from the ab initio calculations (HF/6-311+G(d,p)). The results for the principal distances (r(a)) and angles (angle(h1)) obtained from the combined GED/ab initio (with estimated 1 sigma uncertainties) are r(Ge-Ge) = 2.417(2) angstrom, r(Ge-C) = 1.956(1) angstrom, r(C-H) = 1.097(5) angstrom, angle GeGeC = 110.5(2)degrees, and angle GeCH = 108.8(6)degrees. Theoretical calculations were performed for the related molecules ((CH3)(3)Si-Si(CH3)(3) and (CH3)(3)C-C(CH3)(3)).