Formaldehyde is a HAP specifically targeted for reduction in the US EPA RICE NESHAP rule. Both SCR and PGM-based oxidation catalysts are highly active for formaldehyde oxidation and can enable attainment of the required high removal efficiencies. However, when designing the catalyst system for emissions control on a natural gas firing engine, it is necessary, under certain conditions, to account for the inhibition effect that unburned hydrocarbons (specifically alkenes and alkanes) present in the exhaust gas have on the formaldehyde oxidation rate. This paper will present experimental results that demonstrate that this apparent inhibition effect is due to the catalytic partial oxidation of the unburned hydrocarbons to formaldehyde as a primary product. This formaldehyde formation pathway, which can have an appreciable effect on the catalyst's net removal efficiency for formaldehyde, needs to be considered in the catalyst system design to assure emissions compliance.