The work described in this thesis had two main aims: (1) to develop a method for the determination of ¹³C/¹²C ratios by measuring the areas of individual rotational lines in gas phase spectra; (2) to measure the absolute infrared intensities of the CH stretching bands in the gas phase of various small molecules, especially partially deuterated species containing only one CH bond. Both projects involved the use of a Fourier Transform Infrared spectrometer, (FT-IR). In part one, infrared spectra of CO, HCN, CH₄ and CO₂ were investigated to identify the species most suitable for isotopic abundance determination. Of these, carbon dioxide exhibits the largest shift (ca. 60 cm^-1) and is the easiest to prepare and handle in a vacuum system. Procedures for handling organic and carbonate samples have been developed. The precision, on about 2 mg carbon, was satisfactory with a rsd = 1.2 % at the natural abundance level and a limit of detection of 0.026 atom % ¹³C above the natural abundance level of 1.136 atom % ¹³C. Results obtained by this method were found to be in good agreement with those obtained by Mass spectrometry. In part two, the absolute intensities of the CH stretching bands in the spectra of C₂H₆, C₂D₅G, C₃H₈, CD₃CD₂CD₂H, (CD₃)₂CDH, (CH₃)₃CH, (CD₃)₃CH, (CH₃)₃CD, C₆D₁₁H and CHD₂X (where X = C1, I, NH₂, OH and O-CHD₂) were measured using the pressure broadening technique. Where data was obtained for both deuterated and partially deuterated species, the extent of validity of the additivity of intensity per CH bond was explored. Individual CH band intensitites in the various compounds are compared with results from previous studies, where available. Chemical variations are discussed in terms of the electro-optical parameters approach of Gussoni et al.