The structural parameters of the ground-state geometry of Me3Al-NH3 calculated by various ab initio methods (HF, B3LYP, and MP2) are presented. For the two isotopomers Me3Al-(NH3)-N-14 and Me3Al-(NH3)-N-15, the rotational transitions J = 1 <-- 0 and J = 2 <-- 1 were investigated by Fourier transform microwave spectroscopy (4-12 GHz). All transitions showed a complicated hyperfine structure consisting of a large number of lines, so that only partial assignment of the experimental data was possible. The best fit was achieved for the J = 1 <-- 0 transition of the N-15-marked sample (Me3Al-(NH3)-N-15), for which 17 of 42 observed components could be assigned by assuming a symmetric top with one quadrupole nucleus (Al-27) and three internal-methyl group rotors. The combination of microwave spectroscopy and the calculated geometry of Me3Al-NH3 at the MP2(fc)/6-311G(2d,2p) level resulted in an Al-N bond length of 2.066(1) Angstrom as the best estimate for the experimental value. These results are compared with those of the well-known isomer H3Al-NMe3 (Warner, H. E.; et al. J. Phys. Chem. 1994, 98, 12215. Atwood, J. L.; et al. J. Am. Chem. Sec. 1991, 113, 8183. Almenningen, A.; et al. Acta Chem. Scand. 1972, 26, 3928. March, M. B. C.; et al. J. Phys. Chem. 1995, 9,; 195). The solid-state structure of Me3Al-NH3 was solved from X-ray powder diffraction data. The compound crystallizes in the orthorhombic space group Ama2 with four molecules per unit cell. There are significant differences between the structure of ammonia trimethylalane in the gas phase and in the solid state. The main differences could be understood on the basis of Onsager's theory using SCRF calculations (B3LYP/6-311++G(2d,p)) (Foresman, J. F.; Frisch, AE. Exploring Chemistry with Electronic Structure Methods, 2nd ed.; Gaussian,:Inc.: Pittsburgh, PA, 1996. Onsager, L,. J. Am. Chem. Sec. 1936, 58, 1486).