Study of the ozonation of aqueous solutions of four heterocyclic nitrogen compounds, at slightly acidic pH in a heterogeneous gas-liquid system, showed that the reactivities of these compounds are different. Amitrole, a five-membered ring heterocycle, has been found to be a highly reactive compound, while atrazine, a six-membered ring heterocycle, has been found to be a poorly reactive compound. The benzotriazoles, benz-fused five-membered ring heterocycles, present intermediate reactivities. Moreovoer, the chemical natures of the ozonation by-products have been found to be different between amitrole and atrazine. With atrazine, ozone did not open the heterocyclic ring, and led to the formation of a trioxotriazine. With amitrole, ozone broke the heterocyclic ring and formed mainly formamide.