The application of ultrasonic spectroscopy for particle-size measurements was investigated during the batch crystallization of (L)-glutamic acid a-polymorph from an aqueous solution. The technique, based on measurements of the attenuation of ultrasonic waves through the suspension, was applied using a prototype ultrasonic spectrometer with a flow-through cell. High-precision measurements of ultrasonic attenuation in the frequency range 7-110 MHz were performed, the deconvolution of which enabled in-process measurement of crystal size distribution and solid concentration throughout the crystallization process. In addition to evincing secondary nucleation, growth and crystal breakage on-line in real time, the experimental results were used to obtain kinetic parameters essential for process design, including secondary nucleation rate and growth rate.