The superficial gas velocity is one of the key parameters used to determine the flow hydrodynamics in gas-solids fluidized beds. However, the superficial velocity varies with height in practice, and there is no consistent basis for its specification. Different approaches to determine the superficial gas velocity in a deep gas-solids system are shown to cause difficulties in developing models and in comparing predictions with experimental results. In addition, the reference conditions for superficial gas velocity are important in modeling of deep gas-solids systems where there is a considerable pressure drop. (C) 2011 Elsevier Ltd. All rights reserved.