Future regulations for passenger cars will no longer focus on emission reduction only but also on reducing CO2. The use of Compressed Natural Gas (CNG) in combustion engines is one solution which provides benefits in CO2 and in pollutant emissions at the same time. The conversion of Gasoline engines to CNG operation is well known. In this paper however - the operation of a passenger car diesel engine in Diesel - CNG dual fuel mode is investigated.The paper describes the experimental setup and measurement procedure that was chosen to assess combined Diesel - CNG combustion. Results for emissions, fuel economy (CO2), engine noise and combustion stability will be presented for three different operating points on a research single cylinder engine.Special focus lies on the partially/unburned hydrocarbon (HC) emissions which are typically high when CNG is well premixed and burning in a globally lean combustion environment. Usable CNG / Diesel mixture ratios are discussed for different load points with respect to the tolerable HC emissions and exhaust temperature level at the Diesel oxidation catalyst (DOC). Finally different calibration countermeasures to lower the HC emission are discussed. The paper ends with a summary of the achievable fuel cost and CO2 savings and with an outlook on how to further optimize Diesel - CNG dual fuel combustion systems.