Abstract: A gas plenum assembly for a fluidised bed apparatus, including a structure defining one or more side and base walls about a plenum chamber and means defining an inlet for gas to the chamber, arranged so that the gas flows generally upwardly into the chamber from the inlet. The assembly includes distributor means overlying the chamber, having multiple openings through which the gas exits the chamber for forming a fluidised bed above the distributor means. Means are disposed between the inlet and the distributor means for spreading the gas flow among the openings. The gas flow spreader means has a plurality of apertures for the gas flow therethrough, and the gas flow spreader means and the flow apertures are sized and arranged to substantially reduce cross-flow velocity of the gas at the openings.