An experimental investigation of the effects of separate hydrogen and nitrogen addition on the emissions and combustion of a diesel engine was performed and the results are presented in the current paper. The experiments were carried out on a fully instrumented HSDI diesel engine having separate hydrogen and nitrogen lines connected to the inlet pipe. Bottled gas was inducted into the engine at concentrations from 2 up to 8% (v/v of the total intake charge) in 2% steps. The effect on NOx, smoke and CO emissions was investigated. Hydrogen's combustion efficiency, calculated from the unburned hydrogen in the exhaust gas, is also presented. Furthermore, an efficiency comparison between hydrogen-diesel, nitrogen rich intake charge and baseline operation is demonstrated. The results showed that hydrogen addition decreases smoke and CO at the expense of NOx emissions. An exception was observed at low speed low load operation where NOx emissions maintained at the same levels. Generally, brake thermal efficiency was slightly deteriorated at low speed but slightly improved at high speed operation. Hydrogen combustion efficiency is increased with increasing hydrogen fraction, load and speed. A massive reduction of NOx is achieved by diluting the intake charge with nitrogen, but this comes at the expense of smoke, CO and fuel consumption penalties. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.