The expansion angle theta of gas jets issuing in fluidized beds is one of the parameters necessary for the application of hydrodynamic models of the jet region. A limited body of experimental data is available in the literature so that a method of prediction is still lacking. The phenomena associated with the formation of a jet above an upward gas discharging nozzle in a fluidized bed have been analysed with particular reference to the influence that the operating variables and the geometric configuration have on theta. The analysis allowed the relevant variables concerned with these phenomena to be found and their connection with the dynamic features of the jet and of the bed, contrasting with the current viewpoint that considers theta as a parameter dependent only on the static characteristics of the system. The dimensional analysis yielded a set of relevant dimensionless groups among which the two-phase Froude number provides a good correlation of most of the literature data.