Abstract: A multiple-stage catalytic conversion system in which a hydrocarbonaceous charge stock and hydrogen flow serially through a plurality of catalytic reaction zones in each of which the catalyst particles are movable via gravity-flow. The reaction product effluent from each zone, in admixture with added hydrogen is heated prior to the introduction thereof into the succeeding reaction zone. The proportions of added hydrogen (portions of a recycled hydrogen-rich phase) are such that (i) vapor density decreases, and, (ii) lateral pressure drop across the catalyst bed increases in the direction of reactant stream flow through the plurality of reaction zones. The disclosed technique alleviates problems associated with the occurrence of stagnant catalyst areas as a result of catalyst particles being "pinned" within the reaction zone and thus unable to assume a downward gravity-flow pattern. In addition, the technique allows utilization of excess heater capacities before and between reaction zones.