Renewable natural gas (RNG), produced from biogas upgrading, is an important alternative to fossil fuels. Unfortunately, RNG contains several trace contaminants, one being NH 3 , of particular concern for RNG produced from farming operations. The presence of NH 3 in RNG, particularly if it is injected into the natural gas (NG) pipeline network, could have serious consequences, ranging from damage to the NG infrastructure, to corrosion of the end-user equipment, and increased pollutant formation during combustion. This paper is part of a broader investigation studying all such impacts, the focus here being pollutant emissions. Two common end-user equipment were tested: An internal combustion engine and a household water heater, both operating with NG injected with NH 3 with concentrations ranging from 0 to 500 ppmv. Emissions studied included unburned hydrocarbons (UHC), CO, and nitrogen oxides (NO x ), the latter being of key concern. The presence of NH 3 resulted in increased NO x emissions for both equipment. For the water heater, the relationship between the amount of NO x formed and the NH 3 concentration was quantitative: For every molecule of NH 3 fed to the water heater, one molecule of NO x was additionally produced. However, for the engine a lower quantity of NO x was formed, to that corresponding to complete conversion. The results with both devices are significant, indicating the need for thorough RNG purification prior to its injection into the NG network.