The complex HF—BF3 has been examined by ab initio methods and the rotational spectra of four isotopic species have been observed via pulsed-nozzle Fourier transform microwave spectroscopy. The experimental structure places the fluorine of the HF near the C3 axis of the BF3, with an intermolecular B-F separation of 2.544(2) Å. The proton is off axis, but rapid vibrational averaging produces a complex which is effectively a symmetric top in the ground vibrational state. The average B- -F-H angle determined from the moments of inertia is 104.1(1)°. These results are in excellent agreement with the ab initio calculations, which give a binding energy of 3—4 kcal/mol for the complex and a 2° out-of-plane distortion of the BF3 moiety. Taken together, the results indicate a complex which is essentially weakly bound in nature, though interestingly, the intermolecular B-F distance is slightly shorter than the sum of reasonable van der Waals radii. The complex resembles an incipient donor—acceptor adduct of the kind frequently observed or invoked in solution-phase Friedel-Crafts reactions, and its relationship to such chemistry is discussed. The experiments were performed on a newly constructed instrument featuring direct software-driven control of the pulse timing sequence without the need for home-built digital electronics. The essential characteristics of the apparatus are described.