Nitrogen dioxide (NO 2 ) emission levels of fuel jets were experimentally studied for H 2 , H 2 /He mixtures, a H 2 /He/CH 4 mixture, and CH 4 . The study was undertaken to understand the dependence of NO 2 emission in turbulent diffusion flames on parameters other than the widely known effects of rapid mixing. These parameters are fuel types (CH 4 vs H 2 ), the initial NO level, and the flame temperature. The fuel mixtures were chosen such that these factors could be investigated independently. In all the flames studied, NO 2 /NO x increases with decreasing NO concentration in the flame and with decreasing adiabatic flame temperature. The CH 4 fuel demonstrates a qualitatively different influence on the NO 2 /NO x ratio than H 2 . Its effects are most pronounced when the flame blowout limit is approached. Adding a small amount of CH 4 to the H 2 flames also qualitatively affected the NO 2 /NO x ratio.