The ethanol decomposition process for hydrogen-rich gas synthesis via microwave discharge in ethanol-water mixtures was investigated in this study. Excited active species and gas-phase products based on ethanol's varying volume fraction were detected via optical emission spectroscopy (OES) and mass spectrometry (MS). The correlation between the spectrum signal of the active species and the mass spectrum signal of the gas phase products was quantitatively analyzed. The experimental results showed that the intermediate active species mainly consisted of OH, C-2, CH2, CH, C, and H, and the gas phase products mainly included H-2, CH4, C2H2, CO, C2H4, and CO2. There were significantly positive linear relationships between H or CH2 and H-2, demonstrating that both H and CH2 were important radicals for generating H-2. Based on the experimental results and analyzed from the perspective of thermodynamics, the reactions of H + H (+M) -> H-2 (+M) and H + H + H-2 -> H-2 + H-2, H + H + H2O -> H-2 + H2O, CH2 + CH2 -> H-2 + C2H2 were all likely to produce hydrogen. (C) 2020 Elsevier Ltd. All rights reserved.