This laboratory study was designed to investigate the mechanism of the formation of the OH radicals during the ozonation of humic substances solutions. The evaluation of the formation of these radical species was performed by using 1,1,2-trichloro ethane (TCA) as a probe. The apparent removal of TCA observed during the ozonation treatment implies the production of secondary species that catalyze the generation of OH radicals from ozone. These species are formed by a reaction between H2O2, formed in-situ, and O-3 and by a radical mechanism involving reactive humic substances sites.