A study has been made of the infrared spectra of simple gases under conditions of very high resolution. Most of the measurements have been made in the spectral region between 2000-400/cm. The molecules which have been studied are CS2, ethylene, methylfluoride and formaldehyde-d2. The spectra have been assigned in considerable detail and the wavenumbers of the assigned lines have been analyzed to yield fundamental data on the structure of these molecules. It appears that the results of this study will have not only fundamental importance but also immediate practical value since the infrared spectra are useful for the determination of concentrations of atmospheric pollutants. (Author).