The horizontal gas-solid jet penetration of a gas-solid fluidized bed is studied using electrical capacitance volume tomography (ECVT). ECVT is a non-invasive imaging technique developed by the authors and it involves providing 3D concentration maps from capacitance sensors.. The fluidized bed under investigation is of 0.3m (12 inch) ID with a 0.5m ID disengagement section and has a total height of 2.4m. A gas-solid jet side injection, with 1 inch ID, penetrates the wall of the fluidized bed in the middle of the ECVT test region. Solids in the side injection are supplied through and blown into the fluidized bed using high pressure gas. Fluid catalytic cracking catalysts (Group A) are used in this study. The superficial gas velocity from the distributor of the gas-solid fluidized bed employed ranges from 0.03 to 0.24 m/s and the gas velocity from the side jet penetration ranges from 0 to 16 m/s. In this study, we observe and analyze gas and gas-solid side injection in the bubbling regime of the fluidized bed, variation in bubble shape, formation, and coalescence.