A two-stage mechanistic model (considering expansion and detachment stages) was developed for bubble formation at a single orifice submerged in a gas-solid fluidized bed, including the effect of plenum volume which has been largely overlooked in the fluidization literature. The model predicts the variation of plenum pressure, bubble volume, and orifice flow rate with time in a manner consistent with that previously reported for gas-liquid systems. For small plenum chambers, bubbles form with greater frequency and smaller detachment volumes than bubbles formed with larger plenum chambers. Decreasing plenum volume causes the gas flow rate through the orifice to fluctuate with a lower amplitude and higher frequency. The calculated variation of bubbling frequency with the volume of the gas chamber is in good qualitative and quantitative agreement with the experimental results from both orifice velocity and pressure fluctuations for a single orifice with glass beads of mean diameter 157 mu m and FCC particles of mean diameter 70 mu m. Predictions for FCC particles were closer to the experimental results than for the glass beads. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.