Quantitative laser-induced fluorescence (LIF) measurements of nitric oxide concentrations [NO] have been obtained along the centerline in atmospheric pressure methane-air and ethane-air counterflow diffusion flames. These flames are highly diluted to avoid both soot formation and the influence of radiative heat losses on NO formation, thereby ensuring NO production mostly via the prompt mechanism. Linear LIF measurements of [NO] are corrected for variations in the electronic quenching rate coefficient by using major species profiles generated by an opposed-few flame code and quenching cross-sections for NO available from the literature. Temperature measurements are also made in the methane-air counterflow diffusion flames by using thin SiC filament pyrometry. The excellent agreement between temperature measurements and model predictions verifies the efficacy of a new calibration method developed for thin filament pyrometry. Predictions using the GRI mechanism consistently underpredict peak [NO] in all flames. This result indicates a need for refinement of both the prompt-NO and CH kinetics, especially the rate coefficient for the prompt-NO initiation reaction. A modified rate coefficient proposed for the prompt-NO initiation reaction significantly improves agreement between modeling and measurements in both the methane-air and ethane-air counterflow diffusion flames. The remaining discrepancy in some flames can be attributed to a lack of refinement in the CH chemistry. Overall, the modified rate coefficient proposed here seems to be a good choice over a wide range of strain rates for both methane and ethane fuels.