Microwave plasma steam reforming of ethanol under vortex gas flow and atmospheric pressure conditions has been investigated. The main gas products of the steam reforming are H-2 and CO as detected by mass spectrometry and Fourier transform infrared spectroscopy. A "black" carbon deposit on the wall has been observed. A previously developed theoretical model for ethanol decomposition accounting for the gas thermal balance and the chemical kinetics has been further extended to account for the addition of steam to the argon/ethanol feeding background gas. The mechanisms of ethanol and water decomposition depend on the ethanol/steam ratio, and several hydrogen production regimes have been identified and discussed. An integral reaction scheme for ethanol/water decomposition is suggested. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.