The development of tunable lasers has made possible spectroscopy at a resolution previously unattainable. Although continuous-wave broadly tunable sources in the 10 pm region do not yet exist , waveguide carbon dioxide lase r s offer a useful tunability about a large number of emission lines in this part of the spectrum. The first part of this thesis contains a review ·or waveguide Co2 laser theory and previous wo r k on these devices. A design intended for high resolution spectroscopy is then described. Two of these lasers have been used in a spectrometer , using a saturated fluorescence to provide a stable and calibrated reference and a microprocessor- based system for control and monitoring of the tunable source. The general principles are resolution limits of sub- Doppler saturation spectroscopy are also covered. The second part is a compendium of measurements taken and insights gained using the completed spectrometer. Collisional transitions between the hype fine components of a methyl iodide absorption have been observed . Precise measurements of phosphine absorption frequencies indicate that far - infrared lase r s may be pumped much further from resonance than has been assumed hitherto. Measurements on the allene molecule have provide:: improved constants for the v g= 1 state; the split ting of states by K- or 1- doubling has also been observed, confirming theoretical predictions. Finally, a· postulated Coriolis interaction between two modes of formic acid has been verified and quantified, and improved constants obtained for, both modes in a study bringing together measurements taken using the waveguide laser spectrometer and the microwave and submillimetre measurements of others.