A cold model of a circulating fluidized bed having a two-dimensional riser, with a 12 x 120 mm section and a 6.4 m height, was equipped with a device to inject a lateral gas stream along the riser. The apparatus was operated under conditions ranging from those characteristic of combustors to those of gas-conversion processes. Flow structures in the interaction region between the rising gas-solids suspension and the lateral gas stream were studied by means of a motion analysis system. Three main configurations were identified. The ratio between the momentum of the lateral gas stream and that of the rising suspension was found to be the parameter able to discriminate among the three configurations. A satisfactory agreement was found with mixing data obtained using a cylindrical riser.