The ignition delay time with mixtures of primary reference fuels (PRFs) and hydrogen were measured using a rapid compression machine (RCM), where the concentration of hydrogen was systematically varied. The experiments were performed for compressed temperatures in the range 735-847 K. The fuel/air equivalence ratio was varied to investigate its effects on the ignition delay of n-heptane and iso-octane. Hydrogen was added with hydrogenenergy-share ratios in the range of 0-10%, meaning that the energy of the hydrogen replaced 0-10% of that of either n-heptane or iso-octane. When hydrogen was added to nheptane, there was a significant increase in the ignition delay, especially during the firststage reaction region. This was explained using the chemical kinetic analysis software package CHEMKIN PRO; hydrogen consumed radical species related to the ignition of hydrocarbon fuels. With iso-octane, there was a significant increase in the ignition delay not only in the fist-stage reaction region, but also in second-stage reaction region. The effects of the hydrogen concentration were found to depend on the equivalence ratio and temperature for both fuels. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.