Graft copolymerization of poly(aniline) and poly(o-toluidine) onto poly(ethylene terephthalate) fiber was conducted by using peroxydisulfate as a lone initiator under nitrogen atmosphere at various experimental conditions in aqueous hydrochloric acid medium. The grafting of poly(aniline) and poly(o-toluidine) onto poly(ethylene terephthalate) fiber was verified by recording cyclic voltammetry of the grafted fiber, conductivity measurements, and thermal analysis. Graft parameters-such as % grafting, % efficiency, and the rate of grafting-were followed. Grafting was always accompanied by homopolymerization. The rate of homopolymerization was also followed in all experimental conditions.