A calibration strategy for the continuous monitoring of solvent-mediated phase transition was developed using in situ Raman spectroscopy. Citric acid which exhibits enantiotropy during its anhydrous/monohydrate phase transition was selected as a model organic product. Using 25 samples in suspension, specific calibration of the spectral data was obtained for estimating in-line both the overall solid concentration in suspension and the composition of the solid phase. In addition to such key-measurements, reliable estimates of supersaturation were computed in-line from the mass balance of the solute. In order to validate the technique, anhydrous to monohydrate phase transition experiments were performed in suspensions at 15 degrees C and the kinetic process involved was monitored. Despite the use of various solid concentrations and reactor configurations, both the reproducibility and the reliability of the in situ Raman measurements are shown to be satisfactory.