An experimental study was carried out in order to determine the effect on performance and pollutant emissions of converting an existing heavy-duty diesel engine for alternative fuel use. More specifically, a HD diesel engine used in commercial vehicle applications with Euro II baseline emission level was studied in two ways: on the one hand the diesel engine was converted to a dedicated lean-burn CNG engine and on the other hand the baseline diesel engine was converted to a dual-fuel engine (diesel + LPG) with multi-point LPG injection in the intake cylinder ports. The CNG engine conversion was achieved by means of some important modifications, such as the reduction of the compression ratio by increasing the volume of the combustion chamber in the piston, the design of a spark plug adapter for the installation of the spark plugs in the cylinder head, the design of a gas injection system to attain efficient multi-point gas flow and injection, and the implementation of a complete electronic management system by means of an engineered gas ECU. Concerning the LPG dual-fuel conversion, some minor modifications were made to the diesel engine such as the installation of the gas train components and the implementation of a gas ECU for the management of the gas and diesel injection using some CAN bus J1939 signals. The emission tests carried out were mainly based on R49-02 ECE and R96 UNECE. The results show the feasibility of this kind of engine conversion in terms of its effect on performance and pollutant emissions compared to the baseline diesel engine certification limits, especially reduced PM reaching Euro V level in this kind of contaminant. The maximum LPG substitution ratio reached in this work was around 30%. The results for maximum speed and 1000-metre acceleration driveability tests performed on an international proving ground with the diesel and CNG engine installed in a truck are also given.