In previous papers, a method for the calibration of ATR FTIR measurement of dissolved solid concentration during cooling crystallizations was presented, and efficient basic procedures for the determination of solubility and metastability curves were described. The present paper reports new experimental results. The crystallization of an organic product is studied thanks to on-line FTIR measurements of supersaturation and off-line crystal size distribution (CSD) determinations. The obtained information is shown to potentially allow a deeper examination of major industrial issues such as the reduction of batch-to-batch variations and the improvement of the CSD of the final particles. A particular attention is focused on the determination of appropriate seeding parameters such as the cooling rate of seeded slurries, the temperature of introduction and the amount of seed. The results obtained demonstrate that, although any modelling approach is outside the scope of this paper, the analysis of the measured supersaturation profiles allows some interpretation of mechanisms governing the final CSD, and consequently, the proposal of improved operating parameters.