The heterogeneous hydrolysis/oxidation of zinc vapor is proposed as a promising reaction path for the exothermic step in two-step Zn/ZnO solar thermochemical water and carbon dioxide splitting cycles. This approach circumvents mass transfer limitations encountered in the oxidation of solid or liquid zinc, promising rapid hydrogen/carbon monoxide production rates and complete conversion of zinc. In this paper, a parametric thermodynamic analysis is presented to quantify the penalty of generating zinc vapor as well as the benefit of achieving complete conversion of zinc via the heterogeneous oxidation of zinc vapor. The penalty for generating zinc vapor is a reduction in water splitting efficiency from 36% to 27% and a reduction in carbon dioxide splitting efficiency from 39% to 31%. However, with heat recuperation this penalty can be avoided. The benefit of completely converting zinc via the heterogeneous oxidation of zinc vapor is an increase in efficiency from ∼6% to 27% and 31% for water and carbon dioxide splitting, respectively.