The paper presents the results of our experiments concerning the esterification of Aspen wood (Populus tremila), as sawdust and solid wood, with maleic anhydride (MA). The esterification was carried out through different methods. The influence of the pretreatment of wood by solvent extraction, as well as the effects of maleic anhydride to wood weight ratio and of the temperature and duration of process on the degree of esterification, have been studied. The changes of the chemical structure were estimated by the recording of the IR spectra and some chemical analyses. The different behavior of esterified and control wood against the humidity resulted from some specific tests as the hygroscopicity test and the total immersion in wate at 20℃.