The ATR-FTIR, the attenuated total reflection Fourier transform infrared spectroscopy, provides information on the time variations of many chemical species present in the solution and is used for in-process monitoring and control of solute concentration and supersaturation in batch cooling crystallization. The FTIR spectrometer is provided by ABB Bomen WorkIR coupled to a Dipper-210 ATR immersion probe equipped with a ZnSe conical internal reflection element manufactured by Axiom Analytical Inc. The view shows the system integrated with an HEL 0.5 litre reactor.