Chlorine dioxide delignification (Do) stages performed in 90% ethanol resulted in post-Do kappa numbers that were 2 to 2.5 units lower than those performed in water. In spite of the reduction in kappa number, various gravimetric and U? spectroscopic techniques for measuring residual lignin indicated that the ethanol medium did not significantly enhance D, stage lignin removal. Differences in post-D, kappa numbers between the ethanol-based and aqueous systems were ascribed to how the bleaching medium affects ClO, oxidation of residual lignin. Ethanol-based Do stages resulted in an oxidized lignin that contains fewer muconic acid structures and more quinone structures than the aqueous-based systems. Indirect evidence from sodium hydrosulfite reduction implied that quinone moieties consume less KMnO, than aromatic structures in lignin during the kappa number test.