Nowadays, the development of new recycling processes of plastics is of high priority in the light of its extensive utilization and associated waste generation. Polyethylene terephthalate (PET), due to its chemical and physical features and great stability, is one of the major polymers widely used in packaging manufacturing. Since PET natural decomposition is very slow, recycling processes are the best alternative to reduce PET waste. During the chemical recycling, PET chain is converted back to monomers building blocks [1] and eventual by-products such as co-monomers or side reaction products. PET depolymerization based on glycolysis leads to the production of the monomer BHET (bis(2-hydroxyethyl) terephthalate) (see Figure 1) and certainly some impurities inherited from initial polymers such as polymerization catalyst, plasticizers, co-monomers, colorants. Among purification steps of recovered monomer, crystallization is a key process to return to a food contact compatible polymer. In the present work we are particularly interested in the in situ monitoring crystallization process of BHET by using and optimizing several spectroscopic techniques, i.e. Near Infrared and Raman spectroscopies [2] using chemometric techniques. We seek i/ to follow the evolution of the crystallization media and ii/ to evaluate the impact of known impurities on nucleation frequency and crystal growth kinetics. To complement in situ data, crystallization products will also be analyzed ex situ using HPLC-UV/MS (monomer and impurities quantification), UV-visible spectroscopy, colorimetry, optical microscopy and laser diffraction (particle size distribution and shape). These so-called references analyses will enable to use multivariate analysis methods to develop prediction models from spectra measured in situ. Preliminary results will be presented and discussed.