Injection molding of thermoplastic resins is well known to produce volatile organic compounds which can adversely affect operator health, but there is little understanding of particulate emissions. Recent molding of nanocomposites produced no emission of carbon nanotubes, but emission of organic nanoparticulates of unknown composition. This work is an investigation of those particulates with the gaseous phase emissions of airborne nanoparticles collected during injection molding of neat and additive-filled polypropylene and polycarbonate compounds. The starting materials, collected nanoparticles, and molded parts were analyzed using thermogravimetric analysis, thermogravimetric analysis coupled with Fourier-transform infrared spectroscopy, differential scanning calorimetry, and gas chromatography-mass spectrometry. The results were analyzed to determine the composition of the nanoparticulates and compounds lost from the polypropylene and polycarbonate during injection molding. Characterizing these emissions will allow for a better understanding of the precautions and safety equipment needed during melt processing.