The invention relates to a device for mixing and distributing a gas and a liquid, arranged upstream from a granular bed, comprising a gas and liquid delivery means and a substantially horizontal lower plate, said lower plate being equipped with a plurality of risers evenly distributed over the surface of said plate, each riser comprising an upper part and a lower end communicating with the underside of said plate, the upper part of the risers being provided with at least one lateral orifice. The improvement provided by the invention lies in that the delivery means comprises a substantially horizontal upper plate above which the gas and the liquid are supplied, said plate being provided with means for carrying the liquid to the lower plate and with gas separation and carrying means.