Paper is susceptible to chemical degradation through hydrolysis and oxidation, resulting in embrittlement and failure. Understanding the embrittlement process is important to ensure the preservation and longevity of historical paper-based documents. However, the complex and architectured paper microstructure is a major challenge for fully understanding this process. Two papers with different microstructures were artificially aged under hydrolytic and oxidative exposure conditions, and the consequences of ageing were studied. The fibre embrittlement, the fibre–fibre bonds deterioration, and the evolution of paper microstructure upon ageing are evaluated through macroscopic and localised mechanical tests, as well as through morphological observations at the microscopic scale. It was concluded, from the different tests in the two principal orientations of the paper, that fibre embrittlement plays a more significant role in the embrittlement process than fibre–fibre bonds deterioration. Specifically, the cellulose chain scissions led to fibre embrittlement, irrespective of the oxidative or hydrolytic nature of the chemical degradation mechanism. Furthermore, we identify a critical degree of polymerisation for cellulose (DP_c ~ 750) below which the evolution of mechanical properties accelerates significantly, regardless of the type of mechanical testing performed. Fibre analysis suggests that the decline in fibre resistance results in fractures occurring under stress at weak points of the fibres, such as kinks or twists.