NOx emission levels and the NO2/NOx ratios were studied in swirl-stabilized H-2-CH4 flames and comparison was made with H-2 flames under similar conditions (i.e. system parameters such as Reynolds and swirl numbers, and the recirculation zone residence time). Changing the fuel composition from 100% H-2 to 50% H-2-CH4 was found to have moderate effects on the NOx emission level. The NO2/NOx ratios with CH4 substitution can be significantly higher than pure H-1 flames (about 80-95% for H-2-CH4 flames, compared to 35-50% for H-2 flames), partly explaining its high values in practical gas turbine combustors.