The reactions of two cellulosic materials (an industrial cellulosic by-product from a Kraft pulp mill facility and a reference material) with three coupling agents, used to improve compatibility between cellulosic reinforcements and thermoplastic matrices, were studied by diffuse reflectance Fourier-transform infrared (DRIFT) spectroscopy and diffuse reflectance ultraviolet-visible spectroscopy. A maleated polypropylene wax (Epolene E-43™) and two silanes (N-2-aminoethyl-3-aminopropyltrimethoxysilane and methyltrimethoxysilane) were used as coupling agents. The two cellulosic materials reacted in a similar way and the three coupling agents were covalently bonded to the cellulose. For the aminosilane, a reaction with cellulose involving a fraction of the amino groups was detected. A simple method, based on the analysis of the oxidation kinetics of treated and untreated materials, was developed to compare the degree of cellulose modification achieved by each coupling agent. The analysis revealed that a reduced fraction of the cellulose reactive groups was converted by esterification with the maleated polypropylene. However, when applied in the appropriate conditions, the two silanes converted most of such cellulose reactive groups. Finally, the reaction of cellulose with mixtures of coupling agents was studied. The aminosilane-treated cellulose reacted with the maleated polypropylene with formation of amide links.