The paper presents experimental studies of a dual-fuel diesel engine which was fueled by diesel fuel in to the cylinder and hydrogen to the intake manifold. The effect of co-combustion of hydrogen and diesel fuel on the stability, performance and exhaust gas emission of the test engine was studied. The energy share of hydrogen co-combusted with diesel was changed in the range from 0 (D100) to 30% (DH30). Increasing the hydrogen energy share to 30% in a compression-ignition engine operating at full load resulted in an increase in the maximum combustion pressure (by 13%), the rate of heat release (by 46%) and the rate of pressure rise (by 35%). The addition of hydrogen, due to the increase of the combustion rate and shortening of the total duration of this process, caused an increase in efficiency, by a maximum of 17% for DH14 and DH25. For DH30 there was a significant decrease in engine stability, confirmed mainly by the determined value of the coefficient of covariance in the indicated mean effective pressure. The addition of hydrogen to a compression-ignition engine, up to 25%, resulted in a significant, almost 85% reduction in soot emissions, an almost 57% reduction in carbon monoxide, and an almost 27% reduction in carbon dioxide. The disadvantage of using hydrogen as a fuel for a compression-ignition engine was increase in HC emissions and a significant increase in NO emissions (by over 80% for DH30). (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.