Abstract We demonstrated the potential of terahertz spectroscopy for contactless and nondestructive inline control of polymeric compounding processes. Using a fiber-coupled terahertz spectrometer, we investigated molten polymers and polymeric compounds. On one hand, we studied the influence of temperature and pressure on the terahertz properties of molten polypropylene. On the other hand, various polypropylene–CaCO 3 and PA6–glass-fiber compounds were examined. We showed that the volumetric additive content can be determined from the terahertz refractive index of the compound. Real-time measurements demonstrated the applicability of terahertz technology as a valuable tool for the inline control of production processes.