The effect of hydrogen admixing on self-ignition of homogeneous and hybrid mixtures of heavy hydrocarbons in air is studied theoretically based on the detailed reaction mechanism of n-decane oxidation. Reactivity of hydrogen-containing mixtures is not always higher than that of pure hydrocarbon-air mixtures. At temperatures less than similar to 1050 K, addition of hydrogen to such mixtures increases the self-ignition delay: hydrogen acts as an inhibitor. With the increase of hydrogen content the duration of the blue-flame reaction stage becomes shorter and even degenerates. This is caused by reactions of hydrogen with intermediate products of hydrocarbon oxidation leading to formation of less active species hindering chain branching processes. At temperatures exceeding similar to 1050 K, hydrogen addition decreases the overall self-ignition delay thus indicating that hydrogen acts as a promoter. These finding have to be taken into account when discussing perspectives of practical applications of fuels blended with hydrogen as well as related explosion safety issues. Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.