Homogeneous alkylation of poly(m-phenylene isophthaiamide) (PMIA) was carried out in N-methyl-2-pyrrolidone. Alkyl halides such as benzyl bromide and allyl bromide were reacted with sodium-metallated PMIA. When the Taft equation was applied to this substitution reaction, the linear relationship was apparently obtained between log k/k(0) and sigma*. The molecular weight of alkylated PMIA decreased by sodium-metallation and alkylation reactions. The weight loss of PMIA derivatives on healing increased with an increasing degree of substitution of the benzyl group, but ii mas improved by the reaction of Vie unsaturated alkyl group with 1,2-ethanedithiol.