Laser flash photolysis with excitation at 248 nm was used to study photo,chemically derived changes of carboxymethylcellulose (CMC) in aqueous solutions. Transient absorption spectra of solutions after photolysis revealed a broad band with a maximum of approximately 720 nm, which could be ascribed to the signal of the hydrated electron. The interaction of the hydrated electron with CMC was slow (<10(7) dm(3) mol(-1) s(-1)), but the OH radical, formed by the decomposition of H2O2, reacted with CMC at a high rate constant (9.5-11.0 x 10(8) dm(3) mol(-1) s(-1)). The rate constant. of the reactions of CMC with hydroxyl radicals depended on the conformation of the macromolecules, which was determined by the pH of the solution. Transient absorption was recorded at a wavelength shorter than 370 nm for CMC solutions photolyzed in the presence of H2O2. As a result of OH attack, long-lived radicals were formed on CMC. The recombination of macroradicals led to the formation of crosslinkina bonds between side-chain groups, and as a result of it an insoluble gel arose in low-pH solutions. (C) 2004 Wiley Periodicals, Inc.