Aminoalkylalkoxysilane (AAAS) flexible copolymer networks can be used to deacidify and strengthen paper in a one-step operation. The treatment is versatile and can be tailored to convey a balanced combination of tensile strength and pliability to degraded paper from libraries’ and archives’ collections. The treatment efficiency on bleached pulp and rag or cotton paper has been shown to be always suitable. It is however challenged when the paper contains lignin and is very degraded. The reasons for this are still poorly understood. Indeed, the reactions at room temperature of AAAS with the biopolymers in paper are difficult to characterize. In this work, the hydrolysis and polycondensation reactions of an AAAS monomer (3-aminopropylmethyldiethoxysilane, AM) were closely examined, and the interactions with lignin and cellulose were studied using a modeling approach. To this end, several model organic molecules were used to reproduce the main functional groups of cellulose and lignin. The reactions with AM were studied with attenuated total reflection–Fourier-transform infrared spectroscopy and nuclear magnetic resonance using 1H, 13C, 29Si, and 1H–X (X = 13C, 15N, or 29Si) heteronuclear multiple bond correlation. The results showed that carbonyl and carboxyl groups react with the amine and silicate moieties of AM. These results are paralleled with the physicochemical properties of three oxidized and brittle newsprint papers to explain the variable degree of strengthening observed upon AAAS treatment