Measurements of the composition of stable and unstable species and temperature in laminar, premixed, flat flames of H2-NO2-Ar have been made and compared to the structure calculated with a flame code including detailed chemical kinetics. No previous detailed flame structure measurements and chemical kinetic modeling of this flame have been presented. Similar measurements and calculations are reported for a companion H2-O2-Ar flame in order to provide a comparison to a previously well characterized flame. Measurements were made of stable species concentration by probe sampling and mass spectrometric sample analysis. Measurements of relative OH concentration and of temperature were made by laser-induced fluorescence. Agreement between the measurements and calculations of stable species concentrations in the H2-NO2-Ar flame using existing chemical kinetic mechanisms with no adjustments to the kinetic parameters is discussed. The reaction mechanism is dominated by the three elementary reaction steps given below: