Diesel engine technology has been driven by stringent regulation. To fulfill this demands emission control systems are constantly improving. In this context exhaust gas aftertreatment development assumes an important role however the in-cylinder emissions control is fundamental to comply with future legislations.Among the aftertreatment systems applied to reduce diesel engines combustion products the Selective Catalytic Reduction (SCR), which converts nitrogen oxides, (NO x ) into nitrogen N 2 and water H 2 O via dosing a chemical reducing agent containing ammonia in the exhaust system is an already proven solution for most medium and heavy duty Diesel (MDD and HDD) applications.The current work aims to compare the behavior of different urea and formamide-based reducing agents using a selective catalytic reduction aftertreatment system (ATS) when it comes to NO x reduction. Another purpose of this study in addition to studying the NO x reduction is also to understand the effects or side-effects in other gases like carbon monoxide (CO), carbon dioxide (CO 2 ), hydrocarbon (HC), oxygen (O 2 ), ammonia (NH 3 ) and to discuss in detail the potential of using this alternative reducing agents to comply with EURO VI standards and optimized SCR systems towards increasing their efficiency.Regarding the SCR catalyst system as to NO x reduction it is possible to say that urea-based mixtures are the most efficient ones but on the other hand are also those that present higher values of ammonia slip. The formamide-based mixtures do not present efficiency as significant as the urea-based mixtures but the ammonia slip levels produced by those mixtures are virtually none.With regard to CO and HC emissions the formamide-based mixtures have presented significant increase on the emissions of those two gases while in the urea-based mixtures the values have remained virtually unchanged.