In a recent paper Rowe et al [ 11 present data on the voldage of the dense phase m a bubblmg flm&zed bed of catalyst powder They find that as the superlicud gas velocity 1s mcreased between 1 and 20 cm/s the voldage of the dense phase at first falls rapldly and then gradually mcreases They also draw some conciusrons concemmg the effect of fines on the dense phase voidage We have been makmg measurements of the dense phase voidage m bubblmg and non-bubblmg beds of fine pmcles usmg the bed collapse method[2]. and although we shall be reportmg our results m fuil shortly, we felt it would be helpful to comment duectly on surulanties and dtierences between our results and those of the authors 1 Our results also show the same general trend m which e,, the dense phase voldage, at first decreases and then gradually mcreases with mcreasmg gas velocity Other workers [2,3] have also commented on this 2 The authors report that e, mcreases with mcreasmg fines content, at constant gas velocity Agam we agree, but would hke to pomt out that m changmg the fines content by elutlation, the authors simultaneously mcreased the mean size of the powder, reduced the fraction of fines ~45 pm, and reduced the particle size dlsmbutlon It IS not easy to tador powders m such a way that the effect of these three vanables can be isolated However, our experiments and others recently pubhshed[3,4] suggest that all these separate effects are unportant 3 We have reason to believe that the values of the dense phase voldages quoted m the paper (0 31-O 38) are much too low We have flmdlzed a great many fine powders sunllar to that used by the authors and have found voldages rangmg from 045 to 065 depending on the mean sue, shape and size dlsmbutlon of the powder Determmatton of the bed voidage depends on knowing the particle densay, and this IS not easy to measure d the pmcles are porous Usmg the mclptent flmduatlon velocity quoted and a correlation which fits a wide range of fine powdersI we estunate the density of the catalyst used by the authors to be approximately 2 g/cm3 This value gves packed bed voidages m the range 0 54 6, much nearer values we have found usmg powders of sumlar size and sue dlsmbutlon These h&er voulages would resolve tbe ddliculty menuoned m