Experiments have been done on the de-aeration characteristics of 18 powders with mean sizes of 3-125 pm and densities 3645000 kgm-'. Powders in Geldart's Group A de-aerate at a constant rate analogous to sedimentation in a liquid, and a standard&d de-aeration time has been correlated with the powder and gas properties. Cohesive Group C powders de-aerate exponentially with time similar to the final solids consolidation stage observed in solid/liquid systems. lNTRODUCTlON The bed collapse technique (Rietema, 1967; Morooka and Kate, 1973) has been used by several workers (Abrahamsen and Geldart, 1980a; Dry et al., 1983; Simone and Harriott, 1980; Tung and Kwauk, 1982) to determine the average voidage, &,, of the dense phase in bubbling beds of Group A powders. Some workers (Drinkenburg and Rietema, 1973; Bohle and van Swaaij, 1978) have assumed that the de-aeration rate, U,, is equal to the superficial gas velocity in the dense phase of the bubbling bed, UD. Collapse tests were done by Sutton and Richmond (1973) in connection with work aimed at using aeration to improve the storage of fine powders in hoppers. Murfitt and Bransby (1980) also worked on deaeration of powders in hoppers but made measurements for only the first few seconds after the air supply was cut off. The hydrodynamic properties (e.g. fluidity) of crackicg and other catalysts are undoubtedly influenced by the particle characteristics, and if these change in use this can give rise to flow problems in standpipes and cyclone diplens. There is therefore increasing interest in tests ;hich can give information on behaviour of aerated powders.