A counterflow configuration was used to measure thermal and species structure in water-vapor diluted nonpremixed methane-air flames. The motivation is to understand the chemical and thermal effects that water has when it is introduced as a diluent into the fuel side. This work is relevant to combustion processes where water is incorporated naturally in the fuel; e.g., methane hydrates, and when water is added intentionally for emission reduction such as in flares and H2O/fuel emulsions combustion. Experimental data are compared to 1-D computations. The agreement is generally very good, but the one dimensional counterflow diffusion model overpredicts flame temperature and major radical, OH, concentration very near extinction in highly diluted H2O-methane/air diffusion flames. Changes in flame position, flame width, and peak temperature with the addition of water were measured. Flame temperatures were measured with thin filament pyrometry. OH-PLIF is used to characterize the flame reaction zone with water dilution; the OH distribution, flame position and thickness from the OH-PLIF images were measured. The results show that the OH intensity and reaction zone thickness decreases with the increase in water. Predictions and experiments demonstrate that water mainly acts thermally to lower the flame temperature until extinction. The OH maximum intensity shifts towards the air side of the counterflow burner with water addition. OH is also measured with CO2 dilution of the fuel stream, and the results are compared with H2O addition, including comparisons with the OH molar peak predictions obtained using the GRI 3.0 mechanism and the CHEMKIN Pro one dimensional counterflow model. The study indicates that water's chemical effects are to change the production and depletion of OH, H and O radicals, especially near extinction. Chemical kinetics simulation of the flame demonstrates good agreement in OH and flame temperatures over a wide range of dilution away from extinction, particularly for CO2. An over prediction of the water carrying capacity near extinction is found for highly water-diluted flames. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.