The role of hydrogen addition on the structure of the Moderate or Intense Low oxygen Dilution (MILD) combustion regime is examined using a combination of experimental techniques and laminar flame calculations. Laser diagnostic imaging is used to simultaneously reveal the in situ distribution of the hydroxyl radical (OH), formaldehyde (H2CO), and temperature using the Jet in Hot Coflow (JHC) burner. The fuels considered are natural gas, ethylene, and LPG (each diluted with hydrogen 1:1 by volume). Hydrogen addition to the primary fuel was found necessary to stabilise the flames. Further to the role of hydrogen in the stabilisation of the flames, hydrogen addition also leads to the reaction zone exhibiting similar structure for different primary fuel types. The independence of the reaction zone structure with hydrogen addition suggests that a wide variety of fuels may be usable for achieving MILD combustion. (c) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.