During the manufacturing of active pharmaceutical ingredients crystalline products in the purest forms are required. Quite often multiple components are present during crystallisation, which requires their continuous monitoring and finally separation. The current work demonstrates the application of ATR-UV/vis spectroscopy with suitable calibration model for the monitoring of a multi-component crystallisation system including both cooling and anti-solvent operations. ATR-UV/vis spectroscopy was used together with focused beam reflectance measurement (FBRM) to monitor the liquid and solid phases, respectively, during the crystallisation of a mixture of ortho- and para-aminobenzoic acid. The model developed using principal component artificial neural networks accurately predicted the concentrations of multiple components present in the mixture. (C) 2012 Elsevier Ltd. All rights reserved.