The present study investigates the deacidification of papers with hexamethylenetetramine (HMT) in alcohols and supercritical carbon dioxide. The effects of various treatments, including HMT-IM (treated by immersion), HMT-SC (treated by supercritical carbon dioxide) and artificial accelerated aging, on the pH and mechanical properties of the papers were studied. Results showed that deacidification provides a sufficient alkaline reserve in the order of HMT-IM >HMT-SC (SCCO2) > HMT-SC (aged) to protect papers against further acid attack. Surface pH values were also apparently enhanced to a reasonable range (pH = 6.7-7.0) after treatment. Deacidification significantly improved the tensile strength, folding endurance index (double folds number) and tear index of papers. In particular, SEM results proved that the tensile strength and folding endurance of papers treated by HMT-SC were better than those treated by HMT-IM. Even after accelerated aging, the surface pH and mechanical properties of the papers remained within proper ranges, thereby benefitting the long-term preservation of aged papers. EDS analysis of elemental N further proved that an HMT reserve in papers is helpful for their long-term preservation. In other words, HMT is a favorable organic deacidification agent in alcohols and supercritical carbon dioxide.