T he use of ozone to attack soluble or solid recalcitrant compounds is often necessary to comply with environmental standards. In order to optimize ozone dosage its action must be properly targeted. Competition for ozone between soluble compounds and solid particles was studied using off-gas ozone concentration profiles. Deduced gas absorption enhancement for each matter type, alone and mixed, indicated non-classical competition. Results can be explained by the mass transfer film concept in the presence of reactive particles. Besides classical influential parameters (concentration and reactivity towards ozone), the temporary size distribution of solid particles related to the thickness of the effective liquid film must be considered.