After thermal aging, the pH values and degree of polymerization (DP) of untreated and treated typographic papers dated to 1946,3 and 2005 were determined. Results indicate that the pH values and DP of treated paper from 1946 is the lowest, and that deacidification improves its durability. It is not recommended that paper of low acidity (high pH) be subjected to deacidification. Rather it should be conserved under a constantly stable environmental condition.