Probe sampling measurements of the concentrations of O-2, CO2, CO, and NO in the postflame zone of the methane-air flames are reported. A heat flux method was used for stabilization of nonstretched flames on a perforated plate burner at 1 atm, Major attention in this work has been paid to the identification of possible uncertainties and errors of the measurements. Radial and axial profiles of the concentrations of stable species and NO in the postflame zone were used to evaluate the influence of the ambient air entrainment, flame expansion, and downstream heat losses. In the core region of the flames, the radial profiles of the major species and NO are flat from moderately lean to moderately rich mixtures. In the very lean mixtures an ambient air dilutes the burnt gases, while in the very rich mixtures it causes oxidation of the combustion products. The buoyancy and radial flame expansion can significantly modify observed concentration gradients in the postflame region. In the methane-air mixtures, the NO concentrations measured at a fixed distance from the burner as a function of stoichiometric ratio were obtained. These dependencies clearly possess two maxima: in stoichiometric mixtures due to thermal-NO mechanism and in rich mixtures at equivalence ratio around 1.3 due to prompt-NO mechanism, The numerical predictions of the concentrations of O-2, CO2., CO, and NO in the postflame zone are in a good agreement with the experiment when downstream heat losses to the environment are taken into account.