Influence of sodium in ferrierite (HFER) zeolite on selective catalytic reduction of NO by acetylene (C2H2-SCR) was investigated. NOx -TPD and FT-IR indicated that small amount of sodium exchanged into the proton-form zeolite with an exchange level of about 11.8% is beneficial for the title reaction by accelerating active nitrate species formation on catalyst surface from NO2 and by suppressing the reductant combustion. Nevertheless, no further improved catalytic performance in C2H2-SCR could be observed by a larger amount of sodium exchanged into HFER due to some inactive nitrate species formed on the zeolite. Instead, activity of the zeolite for C2H2-SCR was drastically reduced, since the capacity of the zeolite for catalyzing NO oxidation and accelerating active NO+ species formation was remarkably depressed.