The objective of this study was to investigate the closed-loop control strategy for a batch crystallizer using two different control methods. The isothermal crystallization of paracetamol (PA) in isopropanol-water mixtures at 20 degrees C was used as the model system. The annuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and the focus beam reflectance measurement (FBRM) were used for in situ measurement of the concentration of paracetamol and chord length counts, respectively. The controlled variables were the supersaturation and the difference in the chord length counts between two sampling intervals and the manipulated variable was the anti-solvent (water) flow rate. The rigid logic control (RLC) and the fuzzy logic control (FLC) were applied. The direct objective of this study was to maintain the controlled variables inside their predetermined ranges. The indirect objectives were minimization of the batch time and maximizing the final crystals size and they were closely related to the meeting of the direct objectives. The experiments conducted with FLC resulted in approximately 30% reduction of time of the experiment and an increase of 80 pm in the final volume weighted mean size. (c) 2007 Elsevier Ltd. All rights reserved.