The role of chlorine (III) compounds, i.e. chlorous acid (HClO(2)) and chlorite (ClO(2)(-)), in chlorine dioxide bleaching were investigated by treating different pulps with Cl(III). It was discovered that in addition to its fully inorganic reactions, chlorous acid is consumed by organic structures present in the pulp. These structures were assumed to be aldehydes. The aldehydes might exist already in unbleached pulps, but chlorine dioxide bleaching was also found to generate new aldehyde groups. The reactive groups were concluded to originate both from carbohydrates and lignin.