The mechanism of chlorine dioxide (ClO2) decomposition in an alkaline medium has been investigated. The formation of radicals and chlorinated species was studied in aqueous solutions containing ClO2 and simple model compounds of lignin or cellulose (vanillin, vanillyl alcohol, veratryl alcohol, methylglucoside and cellobiose) at acidic and alkaline pHs. Because hypochlorite (ClO−) is an intermediate occurring in the course of the reaction mechanism, similar experiments were carried out with solutions of sodium hypochlorite (NaClO) at alkaline and acidic pHs. Electron paramagnetic resonance (EPR) spectroscopy based on the spin-trapping technique revealed the presence of hydroxyl radicals (HO˙) at alkaline pH with ClO2 alone or with model compounds. At the same pH, only a small amount of HO˙ was detected with ClO−. Chlorite (ClO2−) and chlorate (ClO3−) ions were dosed with iodometric titrations, both during ClO2 alkaline decomposition and during reactions with model compounds. Vanillin and vanillyl alcohol were oxidized by ClO2. The intermediate ClO2− was either inert or reacted with the aldehyde function of vanillin. Cellobiose was attacked only in an alkaline medium, either directly by ClO2 or indirectly by HO˙ radicals. This resulted in the formation of glucose, which was then degraded by ClO2− ions. The generation of HO˙ could be one reason for cellulose degradation by ClO2 at alkaline pH, but possibly not the unique one, as it was not proved in this article, whether or not ClO2 is able to directly attach the OH functions of anhydrosugars at alkaline pH.