Poly(ethylene terephthalate) (PET) fibers were grafted with 4-vinyl pyridine (4-VP) using benzoyl peroxide (Bz(2)O(2)) as initiator. Fibers were swelled in dichloroethane (DCE) for 2 h at 90degreesC to promote the incorporation and the subsequent polymerization of 4-VP onto PET fibers. Variations of graft yield with time, temperature, initiator and monomer concentrations were investigated. Percent grafting was enhanced significantly by increasing Bz(2)O(2) concentration up to 2x 10(-3) mol/L and then decreased upon further increase in initiator concentration. Increasing the 4-VP concentration up to 0.6 mol/L improves the graft yield significantly. The optimum temperature and polymerization time was found to be 50degreesC and 2 h respectively. Further changes in properties of grafted PET fibers such as moisture absorption capacity and intrinsic viscosity were determined. Grafted PET fibers were charactarized by thermogravimetric analysis and scanning electron microscopy (SEM). Molar mass of the grafted chains was also determined.