In this study, the grafting of methyl methacrylate onto poly(ethylene terephthalate) fibers using azobisisobutyronitrile is reported. An increase in temperature and monomer concentration were found to increase the graft yield. The graft yield increased up to an initiator concentration of 0.90 X 10(-2) mol/L and decreased at higher initiator concentrations. The fiber densities decreased, and fiber diameters and intrinsic viscosities increased with increasing graft yield. Moisture regain values showed a slight increase with the grafting. Fiber orientation decreased the graft yield to an important extent. The overall activation energy for grafting was calculated to be 11.9 kcal/mol. (C) 1993 John Wiley & Sons, Inc.