In studying heterophase alkaline hydrolysis of ester groups in graft PMMA chains, the role of the structure of the polymer in the macromolecular reaction in the solid phase was revealed. A decrease in the degree of conversion of graft PMMA with an increase in its content in PCA—PMMA graft copolymer, caused by a change in the packing density of the graft chains, was established. The highest degree of saponification was attained in using graft copolymers containing 25–35% graft PMMA. Chemisorption, carboxyl-containing polycaproamide fibre with a SEC for NaOH of 1.3 mmole/g was obtained as a result of saponification of PCA—PMMA graft copolymers.