The level of paper stability was studied during processing of the three kinds of paper with four stabilizing solutions. It is found out that the index of folding endurance retention equal to 0.5 for paper made of cotton cellulose, and subjected to artificial heat aging at 102± 2° C during 12 days can serve as a measure of paper stability after stabilization and restoration processes. In addition it is ascertained that paper samples made of cotton cellulose stabilize at 300° C in the process of pyrolysis when thermoanalyzing. Thus, two criteria of paper stability -- stability indexes -- are suggested as a result of the investigation conducted. The first, on the basis of the folding endurance value determined during the artificial heat aging, and the second, with the thermogravimetric curves used. -- AATA