The solar thermochemical production of H-2 and CO (syngas) from H2O and CO2 is examined via a two-step cycle based on Zn/ZnO redox reactions. The first, endothermic step is the thermolysis of the ZnO driven by concentrated solar energy. The second, nonsolar step is the exothermic reaction of Zn with a mixture of H2O and CO2 yielding syngas and ZnO; the latter is recycled to the first step. A series of experimental runs of the second step was carried out in a packed-bed reactor where ZnO particles provided an effective inert support for preventing sintering and enabling simple and complete recycling to the first, solar step. Experimentation was performed for Zn mass fractions in the range of 33-67 wt % Zn-ZnO, and inlet gas concentrations in the range 0-75% H2O-CO2, yielding molar Zn-to-ZnO conversions up to 91%. A 25 wt % Zn-ZnO sample mixture produced from the solar thermolysis of ZnO was tested in the same reactor setup and exhibited high reactivity and conversions up to 96%. (C) 2011 American Institute of Chemical Engineers AIChE J, 58: 625-631, 2012