In order to protect books made of acid papers from deterioration, the authors conducted neutralization of acid papers with diethyl zinc (DEZ) in gaseous state in the following way: tests samples of newspaper 148x210 mm in size and 9-12 mm thick were set in a vacuum chamber, warmed to 50°C and evacuated for 72 hours. DEZ equal to about 4.0% of the weight of the testpapers was put into the chamber and the papers were kept for 72 hours. After neutralization of the acid papers, the excess DEZ was destroyed with methanol vapor of twice the volume of DEZ, and the chamber was kept for three hours at 50°C, and then it was evacuated again and filled with nitrogen gas. It was confirmed that the acid paper samples were safely and completely neutralized to the core by this process. -- AATA