The paper describes an experimental investigation of the thermal and chemical processes influencing NOx reduction by natural gas reburning in a two-dimensional pilot-scale combustion system. Reburning effectiveness for initial NOx levels of 50-500 ppm and reburn stoichiometric ratios of 0.75-1.0 is evaluated in the low temperature environment (< 1500K) and simple aerodynamics of a firetube package boiler. Both input/out and in-flame sampling trials are reported to provide insight into the controlling kinetic mechanisms. An overall reaction order between 1.5 and 1.6 was observed with respect to initial NO. N2 formation is observed to occur in both the fuel-rich reburning and fuel-lean burnout zones. NO destruction in the reburning zone is attributed to the reaction CH + NO yields HCN + O; < 1 ppm CH radical is required to produce the observed trends in nitrogenous species. Burnout zone N2 formation is influenced by total fixed nitrogen speciation with low fractional conversion of HCN and NH3. Under low primary NO conditions, the reaction CH + N2 yields HCN + N becomes important and is a contributor to reduced reburning effectiveness.