During the emplacement and cooling of igneous bodies, fluidization of wet sediment by water vapour may occur by heating, or by pressure relief during the opening of fractures. Momentary fluidization causes sediment reconstitution and local transport. Continuous fluidization can result in substantial sediment displacement. Theoretical considerations show that fluidization due to heating is not likely to occur at depths where pressure is much greater than 312 bars. Studies of the emplacement of sills of andesite (Ayrshire), fluid rhyolite (Ramsey Island) and viscous rhyolite (Moelwyn Hills) recognize a wide range of phenomena attributable to wet sediment fluidization and in places modify existing interpretations. The transgressive bases of the subaqueously welded ash-flow tuffs of the Capel Curig Volcanic Formation are consistent with emplacement attended by fluidization of the subjacent sediment. Evidently fluidization is a means by which large volumes of wet sediment can be replaced by igneous material with minimal disturbance of the remaining host. Also, fluidization can result in exceptionally complex contact relationships between sedimentary host and intrusive rocks, and can cause fractures formed during cooling to be thoroughly pervaded by sediment. Fluidization is commonly associated with the formation of peperites.