Nitrogen dioxide (NO2) emission levels of fuel jets were experimentally studied for H2, H2/He mixtures, a H2/He/CH4 mixture, and CH4. The study was undertaken to understand the dependence of NO2 emission in turbulent diffusion flames on parameters other than the widely known effects of rapid mixing. These parameters are fuel types (CH4 vs H2), 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, NO2/NOx increases with decreasing NO concentration in the flame and with decreasing adiabatic flame temperature. The CH4 fuel demonstrates a qualitatively different influence on the NO2/NOx ratio than H2. Its effects are most pronounced when the flame blowout limit is approached. Adding a small amount of CH4 to the H2 flames also qualitatively affected the NO2/NOx ratio.