A well-defined modification of poly(tetrahydr0furan)s was realized by grafting with maleic anhydride. The grafted products were investigated by Fourier-transform infrared (FTIR) spectroscopy and l3C nuclear magnetic resonance (NMR) spectroscopy. FTIR spectroscopy evidenced a characteristic shift of the carbonyl valence vibrations of cyclic anhydrides to higher wave numbers due to grafting. The grafting of maleic anhydride, which occurred mainly onto poly(tetrahydr0furan) carbons in a-position, was proved by means of I3C NMR spectroscopy. Furthermore, it was found that approximately 10% of all graftings took place onto poly(tetrahydr0furan) carbons in P-position. All the grafted units were monosubstituted succinic anhydride units. Indications for the formation of poly(ma1eic anhydride) graft units were not found. The quantitative determination of grafted anhydride units was carried out by titration after hydrolysis to the acid form. Remarkably high values of percentage grafting up to 20 wt.-Vo of maleic anhydride were obtained using an initiator concentration of only 2.5 mol-Vo related to maleic anhydride. The high values of percentage grafting at an initial maleic anhydride concentration of 20 wt.-Yo were due to a nearly complete conversion of the maleic anhydride. If the initial amount of maleic anhydride was higher than 20 wt.-Vo phase separation took place during the reaction. This phase separation prevented complete conversion of the maleic anhydride.