There has been little research on poly(alky-lene terephthalate) modification by graft copolymerization with vinyl monomers. There is no reported information on graft polymerization in the molten state. In this study, nadic anhydride was grafted onto poly(butylene terephthalate) and poly (trimethylene terephthalate) with a free-radical initiator in an internal mixer. The influence of the monomer and initiator concentrations on the degree of grafting was investigated. The degradation of these polymers was investigated and characterized with their complex melt viscosity. (C) 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1839-1845, 2003.