The affect of distributing gaseous feed along a bubbling fluidized bed of diameter 152 mm was studied with fluid catalytic cracking particles. Steady- and unsteady-state experiments show that the secondary gas concentration profile at the injection level was almost uniform for high secondary and low primary gas velocities. Secondary gas readily mixed within a short distance above the injection level. Backmixing was primarily influenced by the primary feed superficial gas velocity. Regardless of its injection velocity, secondary gas was barely detectable below the injection level at low primary superficial velocities. Axial dispersion and mass transfer decreased significantly when some gas was introduced at higher levels, compared with a bed where all gas entered through the windbox. Two two-phase models are proposed to simulate gas mixing, the first with a step-change in gas flow rate at the injection level, the second accounting for additional mixing in the jet region. (C) 2004 American Institute of Chemical Engineers AIChE J, 50: 922-936,2004