Starting from the premise that the standard deviation, up, of the pressure fluctuations in a fluidized bed is linearly proportional to excess gas velocity, it follows that up is a function of particle size. This relationship has previously been recognised as a potential route to the continuous monitoring of particle size in fluidized bed processing, and is attractive because of the relative ease with which pressure and pressure fluctuations can be measured. Simple expressions with no fitted parameters can be derived for the case where the constant of proportionality between up and excess gas velocity is unaltered by changes in mean particle size. The results of experiments with mixtures of silica sand prepared by mixing two base batches, with a different mean size, in incrementally varying proportions provide good support for the general validity of the model. (c) 2007 Elsevier B.V. All rights reserved.