This paper presents an experimental study of mild flameless combustion regime applied to methane/hydrogen mixtures in a laboratory-scale pilot furnace with or without air preheating. Results show that mild flameless combustion regime is achieved from pure methane to pure hydrogen whatever the CH4/H-2 proportion. The main reaction zone remains lifted from the burner exit, in the mixing layer of fuel and air jets ensuring a large dilution correlated to low NOx emissions whereas CO2 concentrations obviously decrease with hydrogen proportion. A decrease of NOx emissions is measured for larger quantity of hydrogen due mainly to the decrease of prompt NO formation. Without air preheating, a slight increase of the excess air ratio is required to control CO emissions. For pure hydrogen fuel without air preheating, mild flameless combustion regime leads to operating conditions close to a "zero emission furnace", with ultra-low NOx emissions and without any carbonated species emissions. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.