In this work, the thermal Alder-ene functionalization of highly reactive, vinylidene-terminated, polyisobutene (PIB) with stoichiometric amounts of maleic anhydride at 200 • C was investigated by NMR spectroscopy. The thorough interpretation of the complex NMR spectra of PIB derivatives collected at different reaction times (from 4 to 21 h) was based on the combined use of one-and two-dimensional techniques which allowed the precise structure of all the reaction products as well as their relative content to be ascertained. The analysis evidenced the formation of large amounts of derivatives generated by the classic Alder-ene mechanism and involving the more reactive vinylidene 'exo' double bonds of PIB. More interestingly, the investigation also the investigation allowed also to detect the formation of a noteworthy amount of not expected structure generated by the thermal maleination of the less reactive PIB 'endo' β-form provided by the isomerization of residual PIB 'exo' α-form during the functionalization reaction.