A postirradiation process was evaluated in polymerizing vinyl monomers on poly(ethylene terephthalate) (PET) film, fiber, and fabric. The use of DMF, pyridine, and DMSO as swelling agents to facilitate monomer incorporation and effective polymerization were also investigated. The solvents were effective in promoting the incorporation of acrylic acid (AA) in PET film and that of n-vinyl-2-pyrrolidinone (VP) in Dacron 54. AA, a good swelling agent for PET, produced equivalent polymerization yield and moisture regain results with or without any solvent on Dacron 54 and 64. Polymerization yields on films increased with increasing total doses, but those on yarns and fabrics were independent of total dose. The different results obtained on film versus yarn and fabric from solvent treatment and total dose is thought to be due to the different surface-volume ratio of these substrates. The moisture properties of the substrates were dependent mainly upon the monomer type. Among the monomers studied, VP gave highest moisture regain values, followed by AA. The tensile properties of the modified Dacron 54 were not affected. However the breaking elongation of the modified Dacron 64 was slightly lowered by postirradiation polymerization without solvent treatment and was increased when solvent treatment was included.