Nanoparticles are applied in an increasing number of products (e.g. medicine, food and cosmetics). There is a need to real-time monitor the growth of nanoparticles during production inside the chemical reactor. However, standard particle sizing methods such as dynamic light scattering (DLS) and transmission electron microscopy (TEM) cannot do this. A suitable candidate may be ultrasonic transmission spectroscopy (UTS). Here, the sound speed and attenuation are measured of a dispersion and a physics based model is inverted and fitted to the measurements. The measurements have to be done with great accuracy: e.g. one should be able to measure frequency dependent variations in the speed of sound smaller than 0.05%. The measurements also have to be carried out fast enough compared to the time scale of the reaction. Moreover, the inversion has to be able to deal with the changing properties of the solvent during particle formation. Here, we report on the new ultrasound nanoparticle sizer (UNPS) that has these abilities and therefore could successfully monitor the size and concentration growth of nanoparticles during a reaction inside a reactor in real-time.