Chlorous acid is known to react with aldehydes. This study investigated the oxidation kinetics of different aldehydes by chlorous acid. Reaction rates with chlorous acid at 25 °C were obtained for formaldehyde (11.0 M−1 s−1), vanillin (0.59 M−1 s−1), veratraldehyde (1.00 M−1 s−1), benzaldehyde (5.6 M−1 s−1), glucose (3.39 × 10−3 M−1 s−1), glycolaldehyde (39.0 M−1 s−1), and 5-formyl-2-furancarboxylic acid (5.2 M−1 s−1). The activation energies varied between 26 and 63 kJ/mol. The rate-determining step was shown to be the addition of chlorous acid to the aldehyde structure. The practical significance of the Cl(III) reaction with aldehyde structures in chlorine dioxide bleaching was discussed. It was concluded that various aldehydes present in cellulosic pulp are at least partly responsible for the consumption of Cl(III) in chlorine dioxide bleaching.