FT-IR spectroscopy was used to characterize chemically modified wood pulp and to judge the efficiency of different pulp modification processes. Treatment of wood pulp with aliphatic anhydrides was shown to be more effective in a polar solvent such as DMF compared to a non-polar one typified by xylene. The esterification of hydroxyl group associated with the cellulosic material was indicated by the characteristic absorption bands of the resulting cellulosic esters; the degree of hydroxyl conversion was determined by the ratio of peak intensity of hydroxyl and carbonyl stretching vibrations. The so-called “Infrared Acetyl Index” was shown to give a linear correlation with the gravimetrically determined anhydride uptake. Isocyanate modification of wood pulp yielded more complex infrared spectra, because of various secondary reactions of the reactive isocyanate component. Various aromatic isocyanates, used for chemical treatment of the wood pulp, led mainly to urethane formation, as evidenced by the spectra of the modified pulp. The presence of polyurea or urea as the main reaction product of aromatic isocyanates, postulated by a number of authors, could not be confirmed. Nevertheless, phenylisocyanate treatment, at high concentration of isocyanate and without purification of the modified pulp, led to the appearance of at least one more reaction product in the pulp spectra, which is presumed to be either triphenylisocyanurate, the trimerization product of phenylisocyanate, or carbanilide.