The first part of this dissertation covers the microwave spectroscopy of the weakly bound complexes HI-HF, H₂S-SO₂, Ar-H₂S, and N₂O-HF. These molecules were investigated using a Flygare-Balle type pulsed-beam Fabry-Perot Fourier-transform microwave spectrometer. The spectrometer and its construction are described. The HI-HF complex was found to be hydrogen bonded through the hydrogen of the HF. The hydrogen bond length was found to be 2.720A with a structure such that the monomer bonds form an acute angle of 70°. For H₂S-SO₂ a structure with a S-S distance of 3.67A and H-O distances of 3.1A was obtained. It was established that the hydrogens of H₂S are equivalent is this species. A new set of transitions for Ar-HDS were observed indicating that the hydrogens in this molecule are not equivalent. Several new transitions were also observed for Ar-H₂³⁴S, Ar-H₂S and Ar-D₂S. New insight into the structure of the Ar-H₂S molecule was obtained. Several new transitions for the bent isomer of N₂O-HF were measured and a fit of the ground state constants for this species was performed including quartic distortion constants. The resulting fit improves calculated line centers by two orders of magnitude over previous results. The second part of this dissertation covers the infrared spectroscopy of the ν₆ and ν₈ bands of formic acid. Fourier transform data for these bands were obtained at 0.01 cm⁻¹ resolution using the spectrometer at the Kitt Peak National solar observatory. Diode laser data at 0.001 cm⁻¹ was obtained for the ν₈ band. A two state a-type Coriolis coupled Hamiltonian was used to perform a global fit on all the available data. A greatly improved set of spectroscopic parameters for these two bands were obtained. This new set of parameters has allowed several previously unassigned far I.R. laser and I.R. laser saturation lines to be assigned. Using these improved constants, it should be possible to predict the frequencies for several formic acid far I.R. laser transitions which are not yet accurately measured.

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