Hydrogen cyanide (HCN) is a highly poisonous gas that can form through the reaction between formaldehyde and ammonia (NH3) in the exhaust. While this detrimental side reaction has been the subject of recent catalytic studies under selective catalytic reduction conditions, the molecular mechanism leading to the release of HCN remains far from elucidated. Here, we report the detection of the cyanide intermediate species by operando diffuse reflectance infrared spectroscopy, thereby providing unambiguous evidence that the HCN-forming reaction occurs on the catalyst surface. Formaldehyde likely reacts from the gas phase with pre-adsorbed NH3 to form HCN. Although formate species were formed abundantly upon formaldehyde introduction, they were not responsible for the high level of HCN emissions observed. TiO2 alone is sufficient to catalyze the side reaction, but VOx and/or WOx sites increase HCN production by preventing the formation of inactive polyamide species. These mechanistic insights should serve as a basis for understanding the chemistry responsible for the side reaction so that mitigation measures can be put forward.