Despite being widely used in agriculture, food production and environmental monitoring and regarded as on-line chromatograph in petrochemical and biochemical industries, near infrared spectroscopy (NIR) has found difficulties in its application to processes of particle formation through crystallisation or precipitation where solids suspended in solutions cause problems in instrumentation as well as distortion of the spectra. The research work reported here was motivated by the hypothesis that the effect of particles on the NIR spectra in effect brings an opportunity instead: the spectra might contain useful information of both the solid and liquid phases. Through carefully designed experiments using both glutamic acid solutions and slurries of varied solid concentrations and particle size and temperature ranges and with the help of chemometric data analysis, it was found that the NIR spectra clearly contain sensitive information about the size, solid concentration, liquid concentration as well as polymorphs of crystals, providing the possibility of using the instrument for simultaneous measurement of the multiple properties of both phases. (c) 2007 Elsevier B.V. All rights reserved.