In addition to deacidification and alkaline reserve deposition, and in contrast to compounds used in current mass deacidification processes, aminoalkylakoxysilanes (AAAS) also improve the mechanical properties of paper. This simultaneous double effect was demonstrated with aminopropylmethyldiethoxysilane (AMDES). Following treatment with AMDES, papers of various composition exhibited adequate alkaline reserve and pH, as well as a significant increase in their tensile resistance and folding endurance. These properties were partially preserved, together with a moderate molar mass retention, after hygrothermal aging of a model cotton paper. These beneficial effects of the treatment were also found for papers that had been pre-oxidized to various extents (the oxidation was intended to produce a degree of degradation more comparable to old and brittle papers). This strengthening effect of AMDES treatment was, however, found to be more modest for very highly degraded papers on which, moreover, slight yellowing was observed. This yellowing might be due to a reaction between the amine function of AMDES and the carbonyl functions on cellulose, with possible formation of imines, amines, amides, and Maillard reactions products.