The need for a cleaner and less expensive alternative energy source to conventional petroleum fuels for powering the transportation sector has gained increasing attention during the past decade. Special attention has been directed towards natural gas (NG) which has proven to be a viable option due to its clean-burning properties, reduced cost and abundant availability, and therefore, lead to a steady increase in the worldwide vehicle population operated with NG. The heavy-duty vehicle sector has seen the introduction of natural gas first in larger, locally operated fleets, such as transit buses or refuse-haulers. However, with increasing expansion of the NG distribution network more drayage and long-haul fleets are beginning to adopt natural gas as a fuel. Traditionally, natural gas engines are operated over an Otto-cycle employing a stoichiometric combustion strategy, and using sparkplugs to ignite the fuel and a three-way catalyst (TWC) to mitigate emissions of NOx, CO as well as HC. Alternatively, combusting NG in a Diesel engine would result in higher combustion efficiencies, inherent to the Diesel-cycle, thus, reduce fuel consumption and therefore, further amplify the CO2 emissions benefits of NG.In this regard, ‘retrofit kits’ have been developed in order to convert standard Diesel engines into Diesel-NG dual-fueled engines by taking advantage of the auto-ignition properties of Diesel fuel. These technologies basically substitute a given amount of energy delivered by the Diesel fuel with NG that is being injected into the intake-runners leading to the combustion chambers. The NG substitution rate primarily depends on engine operating modes (i.e. speed and torque) and specific engine calibrations and is seamlessly adjustable. Therefore, particulate matter (PM) as well as gaseous emissions are expected to differ when the engine is being operated in dual-fuel mode as compared to conventional diesel fuel operation only.The primary objective of this study was to experimentally investigate the physical and chemical properties of the emissions products from a heavy-duty Diesel engine (HDDE) after conversion to Diesel-CNG dual-fueled operation. To this aim, a 11.9L Mack AC 460P (MY 2005) HDDE equipped with a conversion kit for dual-fuel operation has been evaluated over the 13-mode European Stationary Cycle (ESC) and Federal Test Procedure (FTP) at West Virginia University's Engine and Emissions Research Laboratory (EERL). Particulate matter emissions were sampled using the gravimetric filter method and characterized in terms of particle number concentrations and size distributions utilizing a Differential Mobility Spectrometer (DMS) from Cambustion (model DMS-500). Data were collected using two different sampling systems, namely through a constant volume sampling system (CVS) and a custom-made double mini-dilution system using ejector-type dilutors allowing to sample directly from the exhaust stack. Results showed a significant reduction of oxides of nitrogen and carbon dioxide emission levels at the expenses of hydrocarbons and carbon monoxide which, on the contrary, increased drastically. Particulate matter emission levels experienced an increase when operating the engine in dual-fuel mode. This fact together with the increased emissions of hydrocarbons and carbon monoxide suggested a deterioration of the quality of combustion.