The effect of a phosphonic acid derivative — T-3 fireproofing compound — on the fireproofing indexes of cellulose materials was investigated. It was found that treatment with a 10% solution of T-3 fireproofing compound produces material with a relatively high oxygen index, 32.5%. Complex thermal analysis is used to study the oxidative power of the carbonized residue (CR) formed during thermolysis of cellulose materials modified with T-3 fireproofing compound. It was found that the yield of CR increases and its oxidation rate decreases with an increase in the concentration of T-3 compound. The CR formed, which has high thermal stability, protects the polymer from heat flow and flames.