The paper presents an experimental investigation of hydrogen-diesel fuel co-combustion carried out on a naturally aspirated, direct injection diesel engine. The engine was supplied with a range of hydrogen-diesel fuel mixture proportions to study the effect of hydrogen addition (aspirated with the intake air) on combustion and exhaust emissions. The tests were performed at fixed diesel injection periods, with hydrogen added to vary the engine load between 0 and 6 bar IMEP. In addition, a novel in cylinder gas sampling technique was employed to measure species concentrations in the engine cylinder at two in cylinder locations and at various instants during the combustion process. The results showed a decrease in the particulates, CO and THC emissions and a slight increase in CO2 emissions with the addition of hydrogen, with fixed diesel fuel injection periods. NOx emissions increased steeply with hydrogen addition but only when the combined diesel and hydrogen co-combustion temperatures exceeded the threshold temperature for NOx formation. The in cylinder gas sampling results showed higher NOx levels between adjacent spray cones, in comparison to sampling within an individual spray cone. Copyright (C) 2014, The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications, LLC. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).