In this study a state of the art passenger car natural gas engine was optimised for hydrogen natural gas mixtures and high exhaust gas recirculation (EGR) rates in the part load domain. With optimal combinations of spark timing (ST) and EGR rate the achievements are significant efficiency increase with substantially lower engine-out NOx while total unburned hydrocarbons or CO-engine-out emissions are not affected. Comprehensive investigations of the parameter space using design of experiments (DoE) algorithms provided a complete picture of the potential of such applications. Combustion analysis on the other hand allowed to identify improvements on the basis of accelerated combustion caused by the hydrogen as well as the reduced gas exchange losses due to EGR and associated less required throttling for a given engine output. The best combinations of EGR rate, hydrogen-fraction in the fuel and ST exhibited optimal in-cylinder pressure characteristics accompanied by moderate combustion peak temperatures and low expansion cylinder temperatures. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.