The reaction of cellulose with polyethylenimines (PEIs) was studied using diffuse reflectance ultraviolet-visible (UV-vis) spectroscopy, diffuse reflectance FUR spectroscopy, and standard colorimetry. PEIs were applied from aqueous solutions at the natural pH (pH similar to 11) and at pH = 6. The obtained materials were exposed to different thermal treatments in air. Celluloses treated at pH = 11 suffer the reversible formation of amine bicarbonate salts. This reaction was not observed in celluloses treated at pH = 6 because of the protonation of the amine groups. In all treated celluloses, the PEI amine groups reacted with cellulose carbonyl groups at moderate temperatures to form Schiff bases, which were responsible for the yellowing of the material. At higher temperatures other oxidation products were detected in the UV-vis and infrared spectra. (C) 2004 Wiley Periodicals, Inc.