Experimental measurements of the adiabatic burning velocity in neat and NO formation in CH4 + O2 + Ar flames doped with small amounts of N2O are presented. The oxygen content in the oxidizer was varied from 15 to 17%. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. The Heat Flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + oxygen + argon mixtures were found in satisfactory agreement with the modeling. The NO concentrations in the flames doped with N2O (100 ppm in the argon stream before mixing) were measured in the burnt gases at a fixed distance from the burner using probe sampling. Axial profiles of [NO] were found insensitive to the downstream heat losses. Experimental dependencies of [NO] versus equivalence ratio had a maximum between F= 1.1 and 1.2. Calculated concentrations of NO were in good agreement with the measurements. In lean flames calculated concentrations of NO strongly depends on the rate constant of reaction N2O + O=NO + NO if too high values proposed in the literature are employed. These new experimental data thus allowed for validation of the key reactions of the nitrous oxide mechanism of NO formation in flames. © 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.