A novel method for producing hydrogen with water and BOF steel slag was developed. The steel slag was reacted with water during 2-57 days at 50 MPa for temperatures ranging from 473 to 673 K. The quantitative evolution of the slag and gas compositions indicated that the main H-2 producing reaction is: 3FeO + H2O --> Fe3O4 + H-2 At 523 K, approximately 5 NL of H-2 per kg of slag were produced in 3 days. The reaction rate was only 1.5 times faster when the slag was crushed down to an initial particle size below 50 AM. The H-2 production has been also tested on slags carbonated beforehand at 0.142 +/- 0.002 kg of CO2 per kg of slag. The reaction was found to be thermally activated. A high purity hydrogen (99.995%) is produced with non-carbonated steel slag below 573 K whereas CH4 production was measured in all the other experiments. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.