This study was aimed at experimentally investigating the impact of diesel/natural gas (NG) dual-fuel retrofitting onto gaseous emissions emitted by i) legacy, model year (MY) 2005 heavy-duty engines with cooled EGR and no after-treatment system, and ii) a latest technology engine equipped with DPF and urea-SCR after-treatment systems that is compliant with 2010 US-EPA emissions standards. In particular, two different dual-fuel conversion kits were evaluated in this study with pure methane (CH4) being used as surrogate for natural gas. Experiments were conducted on an engine dynamometer over a 13-mode steady-state test cycle as well as the transient FTP required for engine certification while gaseous emissions were sampled through a CVS system.Tailpipe NOx emissions were observed at a comparable level for diesel and diesel/CH4 dual-fuel operation for the 2010 compliant engine downstream the SCR. However, a reduction of ∼26% in diesel exhaust fluid (DEF) consumption was measured for dual-fuel operation, corroborating with engine-out NOx concentrations that were observed to be ∼32% lower as compared to diesel-only operation, indicating that the addition of natural gas significantly decreased NOx emissions formed during the combustion process. On the other hand, dual-fuel operation of the non after-treatment equipped, legacy engines showed conflicting NOx emissions results. While NOx emissions were observed to reduce for one of the legacy engines by ∼25%, dual-fuel operation of the second legacy engine resulted in a 6.6% increase in brake-specific NOx.Carbon dioxide emissions (CO2) were observed to decrease across all steady-state engine load modes and by 3 to 8% over the transient FTP cycle with no specific trend towards an engine technology. Methane addition to the post-2010 engine, however, resulted in the highest overall CO2 reduction for the given amount of CH4 injected. Additionally, this study was focusing on evaluating possible greenhouse gas benefits (i.e. tank to exhaust) from diesel/NG retrofitting as a function of engine technology. It was observed that despite the reduction in CO2 emissions the global warming potential (GWP) benefits of dual-fuel operation were outweighed by a significant increase in methane-hydrocarbon (MHC) emissions, specifically by factors of 1.6 and 2.3-4.8 for the post-2010 and legacy engines, respectively.