Recently, fourier transform infraRed (FTIR) spectroscopy was reported as a promising technique for in situ measurement of supersaturation during solution crystallization processes. The attenuated total reflection (ATR) probes appeared to be particularly suited to the chemical and physical complexity of industrial suspensions. However. to be used in an industrial contest, the technique must be easily and quickly adaptable to different systems. In order to achieve such an aim, a calibration procedure to monitor supersaturation from FTIR measurements is presented. General comments and recommendations about important technical aspects of the technique are also given. Then the technique is used to develop efficient procedures for the determination of solubility: and metastability curves. The crystallization of three line chemical products is studied, thanks to on-line FTIR measurements of supersaturation and off-line CSD determinations. It is shown that the monitoring of supersaturation is a valuable tool for an improved analysis of key phenomena involved during crystallization processes (primary and secondary nucleation. agglomeration, phase transition, seeding, etc.).