This work presents a general approach to employ different complementary spectroscopic in-situ monitoring techniques in combination with multivariate analysis to estimate liquid and solid compositions in suspensions. The polymorph transformation of L-glutamic acid was selected as a representative process to apply the developed methods. ATR-FTIR and Raman spectroscopy in combination with multivariate analysis were employed to measure solute concentrations and solid composition, respectively. Additionally it was shown Raman spectroscopy can be employed to estimate solute concentrations. The kinetics of the transformation process was determined by combination of a population balance model and a parameter estimation technique. This model can be used to describe the polymorph transformation in a wide range of operating conditions.