Treatment of nitrogen oxides (NO_x) by using a hybrid process consisting of ozonization and catalysis was investigated. The ozonization method may be an alternative for the oxidation of NO to NO₂. It was found that nitricoxide (NO) was easily oxidzed to nitrogen dioxide (NO₂) in the ozonization chamber without using any hydrocarbon additive. In a temperature range of 443 to 503 K, the decomposition of ozone into molecular oxygen was not significant, and one mole of ozone appoximately reacted with one mole of NO. A kinetic study revealed that the oxidation of NO to NO₂ by ozone was very fast, almost completed in a few tens of milliseconds. When the amount of ozone added was less than stoichiometric ratio with respect to the initial concentration of NO, negligible NO₃ and N₂O₅ were N₂ by a catalyst (V₂O₅/TiO₂), indication that the mixture of NO and NO₂ reacts faster than NO.