In this work, three-dimensional numerical simulations with the aim of investigating the mixing of secondary gas in a bubbling fluidized bed are performed. Single and multiple horizontal gas jet injections into a small scale rectangular bubbling fluidized bed are studied. A tracer gas is introduced through the jet orifice to study the gas mixing in the system. Both transient and time-averaged results are analyzed. The effect of gas injection velocity and jet arrangement on the gas-solid contact is evaluated. It is found that the tracer distribution is non-uniform at the injection level. within a short distance above the injection, the tracer becomes uniformly distributed. Gas back-mixing is observed in all simulations, which is prominent near the wall due to the downward flow of solids. For the cases studied, the gas back-mixing tends to decrease as the secondary gas flow rate increases. For the same secondary gas flow rate, it has been demonstrated that a better mixing between the tracer gas and solid particles is achieved when the secondary gas is injected through distributed jet arrangement. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.