Rotational spectra of two isotopomers of the pyridine-hydrogen fluoride complex were observed by using a pulsed-nozzle, FT microwave spectrometer. The constants A 0 , B 0 , C 0 ,Δ J , Δ J K , δ J , Δ K , χ( 1 4 N) were determined for both isotopomers from analysis of the spectra. The complex is assigned C 2 v symmetry, with the HF molecule lying along the C 2 axis of the C 5 H 5 N subunit. The distance r(N...F) = 2.609(5) 9 was determined by fitting the rotational constants under the assumption of unperturbed monomer geometries, and the intermolecular stretching force constant k σ = 27.0(3) N m - 1 was calculated from the distortion constant Δ J . The properties of a series of complexes B...HF, where B is a nitrogen-containing Lewis base, are compared.