Ultrasonic transmission is a simple, non-destructive method to characterize turbid systems like e.g. emulsions or polymer dispersion. Here we use this technique for in-line monitoring of the miniemulsion polymerization of styrene. Either the ultrasound attenuation or the velocity signal provides a fingerprint characterizing the reaction process. During the reaction process the sound velocity increases by about 150 m/s and the attenuation coefficient alpha changes by more than 75%. In the early stage of polymerization the thermoelastic effect dominates alpha and accordingly alpha decreases when more and more polymer chains are generated. In this regime the fractional conversion of monomer to polymer X is obtained from a calibration of the alpha values using gravimetrically determined X data. In the later stage of the process polymer relaxation dominates and alpha goes through a sharp maximum when the reaction temperature matches the dynamic glass transition temperature T-g,T-dyn of the polymer/monomer mixture within the droplets. Then X can be directly calculated from T-g,T-dyn without calibration only using physical material parameters for the given monomer/polymer system, and the corresponding data are in excellent agreement with gravimetrically determined values. This method may be applied to the emulsion polymerization of other monomers, but also for monitoring solution polymerization processes as long as the reaction temperature T-R is below T-g,T-dyn. In case of emulsion polymerization the initial decay of alpha may also be used for process control for systems, which exhibit a change in thermoelastic contrast during polymerization, even if T-R > T-g,T-dyn, but needs additional calibration. (C) 2013 Elsevier B.V. All rights reserved.