This report concerns the application of infrared and ultra-violet absorption spectroscopy to the determination of composition and molecular structure of silicone fluids. The spectra of a number of pure reference compounds of open-chain, cyclic and branched dimethyl siloxanes have been obtained for the first time. The spectral data have been correlated with molecular structure and several useful relationships have been determined. These relationships have been used in a partial interpretation of the spectra of several commercial silicone fluids. The types of polymer structures in each of the silicones examined has been determined, but in some cases the exact structure of the polymer has not been deduced.