This study investigates the early stages of monosulfoaluminate and ettringite carbonation using nondestructive in situ Raman spectroscopy monitoring. Coupling this technique with Principal Components Analysis enables relevant information to be extracted from spectral data such as an accurate identification of the beginning of the reaction and of specific chemical phases. These two hydrates and some of their carbonation products can be well detected using this vibrational spectroscopy technique. Experiments were conducted on two pastes of hydrated yeelimite, in the presence and in the absence of gypsum respectively, during their carbonation. The results confirmed the detection of the beginning of the carbonation and allowed us to identify some consumed and formed chemical phases in the early stages of the reaction. These outcomes of in situ Raman spectroscopy monitoring study on cementitious phases could help understand the early chemical modifications occurring in complex environments such as cement pastes.