A two-step solar thermochemical cycle for splitting CO2 with Zn/ZnO redox reactions is considered, consisting of: (1) the endothermic dissociation of ZnO with concentrated solar radiation as the heat source and (2) the non-solar, exothermic, reduction of CO2 to CO by oxidizing Zn to ZnO; the latter is recycled to the first step. The second step of the cycle is investigated using a packed-bed reactor where micron-sized Zn particles were immobilized in mixtures with submicron-sized ZnO particles. Experimental runs were performed for Zn mass fractions in the range 67-100 wt % and CO2 concentration in the range 25-100%, yielding Zn-to-ZnO conversions up to 71% because of sintering prevention, as corroborated by SEM analysis. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 2529-2534, 2011