The gas-phase adduct CH3CN-BF3 has been observed by rotational spectroscopy. The structure is that of a symmetric top with the nitrogen end of the CH3CN toward the boron. The B-N bond length is 2.011 ± 0.007 Å, and the NBF angle is 95.6 ± 0.6°. The structure is reminiscent of the classical dative bond chemistry known for adducts of BF3 with nitrogen-containing donors, and of related weakly bound complexes of BF3, but is extremely unusual in that the bond length and bond angle are intermediate between the limits normally observed for van der Waals and covalently bonded systems. Moreover, the B-N bond length is 0.381 ± 0.011 Å longer and the N-B-F angle 10 ± 1° smaller than those determined in the solid state by X-ray crystallography, indicating a significant influence of neighboring molecules in the crystal on the structure of a single dimeric unit. We draw analogy with the well-known crystallographic work of Bürgi and Dunitz and co-workers and interpret the structure of this system as a gas-phase snapshot along the reaction path for the formation of the boron-nitrogen dative bond.