Abstract Syngas, an environment friendly alternative gaseous fuel for internal combustion engine operations, consists mainly of carbon monoxide and hydrogen. In this paper, at different loads, the effect of syngas on the performance, combustion and emission characteristics of a diesel engine was studied. For this purpose, three different volumetric compositions of syngas fuels were examined in the diesel engine under dual fuel modes. The syngas with 100% H2 compositions showed an improved engine performance, but, at the expense of higher NOx emissions for an increase in load. The NOx emissions reduced when 25% and 50% CO were added in the 100% H2 composition syngas. At the best efficiency loading point of 80%, the maximum diesel replacement was found as 72.3% for 100% H2 syngas mode. At same engine load, the thermal efficiency was found to be 16.1% for 50% H2 syngas. It increased to 18.3% and 19.8% when H2 content was increased to 75% and 100%, respectively. At higher loads, the 50% and 75% H2 content syngas modes showed a good competitive performance to that of 100% H2 mode. The higher CO and HC emission levels were recorded for 25 and 50% CO fraction syngas fuels due to their CO content in the fuel compositions. At part-loads (20% and 40% loads), all the tested ranges of syngas modes resulted in a poor performance including higher emission levels.