Modelling, design and control of organic batch cooling crystallization processes requires accurate solute concentration and supersaturation measurement. De-supersaturation rate controls the crystallization kinetics and determines the crystal size distribution and product quality. This paper describes a quick, robust and convenient technique for the successful measurement of solute concentration of an organic fine chemical using attenuated total reflectance ultraviolet spectroscopy. The use of multivariate analysis and partial least squares using UMETRICS SIMCA-P software for the calibration and measurement of solute concentration is also described. (c) 2004 Elsevier B.V. All rights reserved.