Since the introduction of Euro IV legislation [1, 2], Selective Catalytic Reduction (SCR) technology using liquid urea injection is (one of) the primary methods for NOx reduction in many applications. Ammonia (NH3) is the reagent and key element for the SCR system and its control calibration to meet all operational requirements. TNO and Horiba are highly motivated to facilitate a correct interpretation and use of emissions measurement data. Different hypotheses were defined to investigate the impact of temperatures and flow rates on urea decomposition. These parameters are known to strongly affect the urea decomposition process, and thus, the formation of NH3. During a test campaign, different SCR catalyst feed gas conditions (mass flow, temperature, species and dosing quantities) were applied. Three Horiba FTIR gas analyzers were installed to simultaneously sample either all upstream or all downstream of the SCR brick. Both steady-state and dynamic responses were evaluated. When undecomposed urea is present, the application of sampling system conditioning above the thermolysis temperature of 133°C can result in additional conversion of urea into NH3. The NH3 concentration reading then does not correctly represent the in-situ condition. It is concluded that a lower sampling system temperature will result in the more accurate and physically sound NH3 measurement. This is an important observation to enable fundamental understanding or development of catalyst and control models. For tailpipe NH3 measurements on full-size SCR systems and typical urea dosing conditions, e.g. type approval measurements, same accuracies are expected for 113oC and 191oC sampling system conditioning.