Gives process for fluidizing solids in a reaction chamber. The catalyst may be uniformly sized and of the same composition, or a number of catalysts may be used so selected and graded that they remain largely in separate layers, although each layer is a part of a common fluidized mass. An Fe oxide or an Fe-Al oxide may be used as the lower layer catalyst with or without Si and a catalyst of lower specific gravity such as Al phosphate, 3NaFˇAlF3 may be used in the upper layer. When the particles are the same size in each layer the lower gravity particles will remain in the upper level and only a relatively thin layer will be mixed with those of the lower layer at an intermediate level. When the particles are all the same density, a double layer can be maintained only when the particles in the lower layer are larger than those in the upper. By adjusting the relative particle size and selecting catalysts of predetermined relative densities, any number of different layers may be maintained. Escape of the top particles may be prevented by making the upper portion of the reactor wider than that adjacent the bottom layer of catalyst. The process may be applied to the Fischer-Tropsch synthesis. 23 United States patent cited.