Raw exhaust gases may contain notable levels of formaldehyde that can negatively impact the efficiency of after -treatment systems. In the selective catalytic reduction (SCR) of NOx over V2O5/WO3-TiO2, formaldehyde was found to react with NH3 to produce HCN at concentrations above the threshold limit value set by environmental/ safety organizations. Due to this side reaction, NH3 is consumed parasitically and the NOx conversion decreases by up to 15 %, even after compensating for the fraction of lost NH3. Under similar conditions, the non-reducible TiO2 support also produced HCN moderately, thereby showing that redox sites promote the reaction but are not a necessary condition. To understand the chemistry responsible for HCN formation, the roles of reaction temperature, water, and oxygen were investigated. Our results suggest a new pathway for HCN production through the direct reaction of formaldehyde and NH3, which is active at high temperature and does not proceed through the formate route previously proposed.