It is known that a large proportion of NO is converted to NO2 through rapid cooling of hot exhaust gas. In order to find a practical method of cooling the exhaust gas and preventing NO-NO2 conversion-especially from unvented space-heaters-the effects or the cooling conditions of the hot exhaust gas on NO-NO2 conversion are fully investigated by both chemical kinetic calculations and experiments. In the calculations assuming various temperature histories, the changes of the chemical species including NO and NO2 were calculated. It was found that the temperature history of the exhaust gas between 1100 and 1400 K seriously affects NO-NO2 conversion. When the exhaust gas is kept near 1100-1400 K for more than 50 ms, the NO2 conversion ratio can be low, even when cooling rate is high. Moreover, detailed kinetic studies show that NO2 formation depends on the balance between formation and decomposition.