The grafting of polyesters onto inorganic fibres by the anionic ring-opening copolymerization of epoxides with cyclic acid anhydrides initiated by potassium carboxylate groups introduced onto the surface was investigated. The introduction of potassium carboxylate groups onto inorganic fibres, such as glass fibre and alumina fibre, was achieved by the treatment of the fibre with 4-trimethoxysilyltetrahydrophthalic anhydride followed by neutralization with potassium carbonate. The anionic ring-opening copolymerization of epoxides with cyclic acid anhydrides was successfully initiated by potassium carboxylate groups on the inorganic fibres and the corresponding polyesters were grafted onto the surfaces; the percentage of grafting of polyester from styrene oxide and phthalic anhydride onto glass fibre reached 36.9%.