The selective catalytic reduction (SCR) of nitric oxide by propene in the presence of oxygen over Pt/ZSM-5 has been studied on samples pre-treated under different conditions. It has been found that reduced samples before SCR of nitric oxide (pre-reduced samples) exhibit a much higher catalytic activity than the oxidized ones before SCR of nitric oxide (preoxidized samples). Moreover, the reactions on pre-reduced samples demonstrate the similarity both in activity and selectivity, whereas different activities have been found on pre-oxidized samples. The rate of NO2 formation rapidly increased above the temperature of maximum catalytic activity for NO reduction. It is therefore suggested that partial reduction occurs on pre-oxidized samples during the NO + C3H6 + O-2 reaction.