The application of ultrasonics to emulsion polymerization reactors is discussed. First, monitoring of conversion in reacting systems by measuring sound velocity at a single relatively low frequency is presented. A simplified modeling approach is adopted, relating the sound speed in the multiphase system to effective values of compressibility and density. The capabilities of such sensors are shown for the emulsion copolymerization of styrene and butylacrylate. Next, the application of sound attenuation measurements to particle sizing is examined. A comprehensive literature model is used to extract the particle size distribution from the measured attenuation spectra for two nonreacting latexes, polyvinylchloride and polytetrafluoroethylene. The problem of model parameter evaluation is addressed, and the results of a parameter sensitivity analysis are briefly discussed. Difficulties related to the presence of ionic emulsifier and large particle concentrations are highlighted.