The methoxyl loss from softwood kraft pulp during chlorine dioxide treatment can be accounted for by methanol and the methoxyl content of dissolved organic material in the bleach filtrate. This was established by a methoxyl group mass balance made at different bleaching times. It was found that ClO2 rather than the in-situ generated hypochlorous acid is mainly responsible for the methanol formation. It was shown that fully methylated pulp is hardly demethylated by ClO2 under industrial ClO2 bleaching conditions when hypochlorous acid is effectively captured. Furthermore, the rates of formation of methanol and methoxyl group containing dissolved organic material are proportional to each other. The experimental evidence suggest that demethylation and delignification are closely related during ClO2 treatment of softwood kraft pulp.