Kinetic ignition enhancement of hydrogen-air diffusion flames by a non-equilibrium plasma discharge of ultra-lean premixtures was studied experimentally through the development of a well defined counterflow system. Measurement and comparison of the ignition temperatures provided a basis for understanding the kinetic enhancement pathways for ignition by plasma discharge of air with and without small concentrations of hydrogen and methane addition. With plasma discharge of the air, the ignition temperatures were significantly decreased when compared to only heated air. The primary species that caused the ignition enhancement was NOx by catalyzing the inactive HO2 radical to OH. With the addition of up to two percent of hydrogen addition to the air upstream of the plasma, the ignition temperature did not significantly change, while only one percent addition of methane yielded ignition temperatures as high as with only heated air. Two pathways of NOx reduction by methane addition were identified where NOx was removed prior to the diffusion flame reaction zone to mitigate the ignition enhancement from the plasma.