Methods and systems for promoting hydrogen gas distribution within cellulosic biomass solids during hydrothermal digestion. On exemplary method can comprise: heating a first portion of cellulosic biomass solids being contacted by a continuous liquid phase and a second portion of cellulosic biomass solids being contacted by a continuous gas phase in the presence of an upwardly directed flow of molecular hydrogen and a slurry catalyst capable of activating molecular hydrogen in the continuous liquid phase; conveying at least a portion of the continuous liquid phase and at least a portion of the slurry catalyst to a location within the continuous gas phase above at least a portion of the cellulosic biomass solids; and after conveying the continuous liquid phase and the slurry catalyst, releasing them such that they contact the second portion of cellulosic biomass solids.