The present paper discusses the fundamentals of specific fuel mixtures as they may be used in lean premix flames in power installations. Experimental measurements of the adiabatic burning velocity and NO formation in (CH4 + CO) + (O2 + N2) flames are presented. The carbon monoxide content in the fuel was varied from 0 to 15%. The oxygen content in the air was varied from 20.9% to 18%. These ranges are typical for addition of reformer gas to methane burners. 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. An overall accuracy of the burning velocities was estimated to be better than ±1 cm/s in the whole range of enrichment by carbon monoxide. The relative accuracy of the equivalence ratio was found to be below 1.6%. Adiabatic burning velocities of methane + carbon monoxide + nitrogen + oxygen mixtures were found in satisfactory agreement with the modeling. The concentrations of O2, CO, CO2 and NO in these flames were measured in the burnt gases at a fixed distance from the burner using probe sampling. Enrichment by carbon monoxide leads to the increase of NO formation in lean and stoichiometric mixtures. Dilution by nitrogen decreases [NO] at any equivalence ratio. Numerical predictions and trends were found in good agreement with the experiments. The results of enrichment by CO on adiabatic burning velocity and nitric oxide formation in methane flames are discussed and compared with similar flames enriched by hydrogen. Experimental peculiarities due to contamination of the fuel mixtures with metal carbonyls are described.