The effects of air-preheat on flame structure are studied in counter-flow methane-air diffusion flames, considering air temperatures in the range 300 to 560 K. Species concentrations for H2, O2, N2, CH4, CO, CO2, C2H2 and C2H4 were measured using sampling and gas chromatography. Concentrations of NO were measured using sampling and a chemiluminescence analyzer. Results of numerical calculations using detailed chemistry were compared with the measurements. The numerical calculations and measurements show good agreement for major species. Independent of temperature in the range 300 to 560 K, measured and predicted concentrations of CH4, CO2, O2 and N2 collapse reasonably well when plotted against mixture fraction. However, the peak CO and H2 concentrations increase with increasing air-preheat. The peak CO concentrations increased by dissociation of CO2. However, the dissociation causes relatively small scatter in the CO2 state relationship. The peak H2 concentrations increase because of an increase in H atom concentrations causing enhanced CH4 dissociation. Both the measured and predicted NO profiles show approximately 70% increase in peak mole fractions with increasing air-preheat. The predictions of peak NO mole fractions are lower by about 10% than the data for TAIR = 300 K and 560 K. However, in the fuel-rich region, the predicted mole fractions are approximately 40% of the data. The increase in peak NO concentration occurs primarily through an enhanced rate of N2 + CH → HCN + N and subsequent oxidation of the N atoms by the OH radical.