A method and apparatus for gas-phase reduction/oxidation is disclosed. The apparatus includes a reactor including at least one reactor tube or containment vessel with active redox material within the reactor tube or containment vessel, a first reactant gas or vacuum for reducing the active redox material, and a second reactant gas for oxidizing the active redox material. The method may be run under substantially isothermal conditions and/or energy supplied to the apparatus may include solar energy, which may be concentrated.