In 1994–1995 the same editions of printed books were treated by four mass deacidification methods: diethyl zinc, Wei T’o, Battelle, and Bookkeeper. Forty-one titles from 1832 to 1991 were sampled from a dissertation collection and four sets sent to mass deacidification plants, while one set was untreated. In 2019, 155 paper samples were analysed for alkaline reserve and pH (cold extract) to compare the performance of treated books with untreated ones. Twenty samples were analysed for degree of polymerization by viscosity average. User experience has also been tested, in 1996 and in 2019. Results showed higher pH and AR for treated books compared to those untreated, and a greater experienced effect for the untreated compared to the treated today. For pH and AR there were differentiations between the deacidification methods, while the results for DP showed greater complexity, pointing to the need for on-going further studies of mass deacidified books.