Cellulose nanocrystals (CNC) are usually obtained by the acid hydrolysis of cellulose fibers with sulfuric acid in harsh conditions. In this process, the amorphous regions are dissolved, and the crystalline part is released, generating sulfated cellulose nanocrystals. To further expand the properties of these CNC, methods to replace sulfated groups for functional groups such as carboxyl,  or grafting polymer onto the CNC surface have been developed. These methods are typically laborious, chemically aggressive, and can be highly polluting. In contrast, milder functionalization methods take place at high reaction times and temperatures. Thus, it is critical to understand the kinetics and overall phenomena occurring during the functionalization of CNC through alternative and more sustainable methodologies. This study presents the reaction kinetics of carboxylation of CNC via a nonaqueous and reactive deep eutectic solvent (DES). This method is based on substituting sulfate groups with carboxylic groups, created by esterification reactions promoted in an oxalic acid-choline chloride (OA-ChCl) DES. Our findings reveal that the ratio C=O/C increases at the expense of S-C/C as the reaction proceeds, followed by X-ray photoelectron spectroscopy (XPS). Likewise, CNC’s chemical characterization, morphology, and crystallinity character were assessed through Attenuated total reflectance spectroscopy (ATR), atomic force microscopy (AFM), and X-ray diffraction (XRD), respectively.).