Solid oxide fuel cells (SOFCs) with direct internal reforming (DIR) provide a promising method to realize clean and efficient utilization of hydrocarbon fuels. Thse endothermic reforming reactions occur simultaneously with exothermic electrochemical reactions at the anode, making thermal neutral state achievable inside a fuel cell, providing reference to the thermal management. In this study, a calculation model combining experimental data and thermodynamic results was established, validating the possibility of achieving thermal neutral state in DIR-SOFCs. In the process of modeling, the electrochemical and thermodynamic characteristics in direct internal steam and dry reforming were elaborately compared, contributing to a more scientific understanding of anode reaction mechanism. Detailed experimental investigation was carried out to determine the influence of H2O/CO2 on the electrochemical properties of DIR-SOFCs, based on which the optimum steam-carbon ratio (S/C) and CO2 to CH4 ratios were obtained. Besides, analysis of distribution of relaxation times (DRT) combined with elementary reactions in CH4-H2O and CH4-CO2 atmospheres were proposed to distinguish different physical and chemical processes within anodes. The results of this study can be conducive to a more precise understanding of reaction mechanism on SOFC anodes and meaningful for practical application of DIR-SOFCs. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.