Internal structure of pure and hydrogen-enriched methane-air flames were investigated experimentally, using simultaneous Planar Laser-Induced Fluorescence of formaldehyde molecule and hydroxyl radical. The fuel-air equivalence ratio and the hydrogen-enrichment percentages were varied from 0.7 to 0.9 and 0% to 50%, respectively. The Karlovitz number was varied between 0.3 and 1, suggesting that the tested experimental conditions pertain to moderate turbulence intensities. A mathematical model that allows for accurate estimation of the heat release rate was developed and guidelines were generated to determine the turbulent flames preheat and reaction layer thicknesses. It was shown that these thicknesses decrease with increasing the fuel-air equivalence ratio and hydrogen-enrichment percentage. Non-flamelet/flamelet behavior of turbulent premixed flames was also investigated using a newly-developed analytical tool. This tool is developed based on the formaldehyde and hydroxyl fluorescence signals deviation from the counterpart synthetic fluorescence signals of the laminar flame simulations. The results show that, for nearly-laminar flames, the flamelet behavior is dominant. However, for both pure and hydrogen-enriched methane-air turbulent premixed flames, deviation from the flamelet behavior (referred to as non-flamelet behavior) is observed. Crown Copyright (C) 2019 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.