The two-step H2O-splitting thermochemical cycle based on the Zn/ZnO redox reactions is considered for solar H? production, comprising the endothermal dissociation of ZnO followed by the exothermal hydrolysis of Zn. A solar-driven thermogravimeter, in which a packed-bed of ZnO particles is directly exposed to concentrated solar radiation at a peak solar concentration ratio of 2400 suns while its weight loss is continuously monitored, was applied to measure the thermal dissociation rate in a set-up closely approximating the heat and mass transfer characteristics of solar reactors. Isothermal thermogravimetric runs were performed in the range 1834-2109 K and fitted to a zero-order Arrhenius rate law with apparent activation energy 361 53 U mol(-1) K-1 and frequency factor 14.03 x 10(6) 2.73 x 10(6) kg m(-2) s(-1). Application of L'vov's kinetic expression for solid decomposition along with a convective mass transport correlation yielded kinetic parameters in close agreement with those derived from experimental data. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 1497-1504, 2009