Nitrate species is crucial intermediate of selective catalytic reduction of NO by acetylene (C2H2-SCR) over HZSM-5. It is proposed that formation of nitrate species on the zeolite is rate-determining step of C2H2-SCR. The proposition is supported by the experimental results: (1) No nitrate species could be detected by in situ FTIR in steady C2H2-SCR over HZSM-5 at 350 degrees C, while the bands due to nitrate species were clearly observed on the zeolite both in situ in NO + O-2 and after a brief evacuation at the temperature; (2) Yttrium incorporation into HZSM-5 zeolite considerably enhanced nitrate species formation on the catalyst and correspondingly the activity of the catalyst for C2H2-SCR was significantly increased. Both the promotional effect of yttrium on C2H2-SCR and the rate-determining step of C2H2-SCR being nitrate species formation over the catalyst were first reported herein.