The combustion of different natural gas-hydrogen mixtures at dry and steam-diluted conditions is investigated for a rich-quench-lean (RQL) and for a premixed combustor. Atmospheric experiments are conducted over a wide range of equivalence ratios and degrees of steam dilution up to a steam-to-air mass ratio of 30%. Emission formation, flame shape and position, and flame stability are measured in combustion tests. An investigation of the influence of steam on the NOx formation is conducted using reactor networks. Steam effectively inhibits the formation of NOx emissions in both combustion systems. For wet conditions, NOx emissions below 10 ppm are achieved for operation on natural gas as well as hydrogen up to high equivalence ratios and temperatures. Increasing the hydrogen content leads to increasing NOx emissions for dry conditions, whereas at wet conditions, NOx is lower for hydrogen-rich fuels. The numerical analysis reveals that in both combustion systems, the thermal, the N2O, and the NNH pathways are significantly constrained at wet conditions. The prompt NOx can be increased due to higher CH radical concentrations for the natural gas flame. CO emissions remain low up to moderate degrees of steam dilution. For a steam content of 20%, the RQL combustor shows increasing CO emissions for low equivalence ratios, whereas they remain low for the premixed combustor. Steam dilution is very effective for NOx reduction in natural gas and in hydrogen flames. Additionally, it lowers the reactivity of hydrogen and already a relatively low steam content prevents flashback. The combination of steam injection and hydrogen combustion provides an efficient and clean combustion mode. (C) 2012 Elsevier B.V. All rights reserved.