The reaction zone of a small-scale laboratory combustor operating under flameless oxidation conditions is examined with the aid of hydroxyl radical chemiluminescence (OH*) imaging and measurements of local mean gas temperatures and local mean major gas species (O-2, CO2, CO, unburnt hydrocarbons and NOx) concentrations along the combustor axis, as a function of the fuel (methane) thermal input, which was varied between 7 and 13 kW. As the fuel thermal input increases, the reaction zone, as typified by the OH* distribution, enlarges and, simultaneously, moves progressively closer to the combustor exit (exhaust) due to the increase in the central jet momentum, while maintaining constant the excess air level. The excess air values used in the present study were low enough to preserve the flameless combustion regime regardless of the fuel thermal input, with the combustor yielding very low NOx (< 6 ppm@15% O-2) and CO emissions (< 14 ppm@15% O-2) regardless of the fuel thermal input. The low NOx emissions, almost independent of the fuel thermal input, are attributed to the suppression of the thermal mechanism promoted by the flameless oxidation regime. Despite being always low, the CO emissions increase with the fuel thermal input presumably because of the lower residence times associated with the higher burner thermal loads. Published by Elsevier B.V.