An apparatus for conducting an endothermic reaction of an organic' compound in the presence of molecular hydrogen and of multicomponent solids is described. The process of operating the apparatus comprises contacting the compound with a solid catalyst for the endothermic reaction and a hydrogen oxidizing solid reagent intermixed with the solid catalyst. Organic products of the endothermic reaction are produced, with evolution of molecular hydrogen. The solid catalyst becomes gradually deactivated by formation of carbonaceous deposits thereon. The evolved hydrogen undergoes an exothermic reaction with the hydrogen oxidizing solid reagent to form a reduction product which comprises deactivated hydrogen oxidizing solid reagent. The deactivated solid catalyst is reactivated by combustion of carbonaceous deposits thereon and the deactivated hydrogen oxidizing solid reagent is reactivated by contacting the reagent with an oxidizing agent in the absence of substantial quantities of hydrogen and in the absence of substantial quantities of organic compounds other than those on the surface of the reagent. The apparatus provides an endothermic reaction which is carried out in the presence of a fluidized bed of catalyst and in the presence of particles of granular hydrogen oxidizing solid reagent which move downwardly through the fluidized catalyst bed, and in which the solid catalyst and solid reagent are separated prior to reactivation thereof in separate reactivation zones.