\Hydrogen production by steam-hydrolysis of zinc is investigated as part of a two-step water-splitting thermochemical cycle based on ZnO/Zn redox reactions. The hydrolysis reactor consists of a hot-wall tube containing a flow of Zn(g) that is steam-quenched to co-produce H-2 and Zn/ZnO nanoparticles. The effects of the quenching gas flow rate and reactor wall temperature on the Zn-to-ZnO chemical conversion and particle yield are examined. Solid products are characterized by X-ray diffraction, N-2 adsorption, and SEM microscopy. Quench rates of 2-6x10(4) K/s yielded conversions of up to 95% at the expense of low particle yield due to significant wall deposition with subsequent hydrolysis. Aerosol particles with hexagonal structure were formed by Zn evaporation-condensation containing low ZnO mass fraction. In contrast, operation at quench rates up to 10(6) K/s led to increased particle yield but lower conversion. Filamentary and rod-like particles were formed with high ZnO content by surface reaction and coagulation. (C) 2008 Elsevier Ltd. All rights reserved.