Much of the fundamental research reported in the literature on gas-solid fluidization properties has been performed with large gas-solid fluidized beds. However, little is known regarding gas-solid fluidization in the mini- and microscale channel sizes ranging from 10(-3) to 10(-2) m and 10(-5) to 10(-4) m, respectively. The wall effects in the mini- and microchannels significantly affect the hydrodynamics of gas-solid fluidization. Such effects are examined experimentally in this study using FCC particles in six mini- and microchannels with sizes ranging from 700 mu m to 5 mm. The data reveal a significant increase in the minimum fluidization and bubbling velocities as well as the wall friction in the mini- and microchannels compared to those in large fluidized beds. Additionally, the maximum stable bubble size increases With the superficial gas velocity and channel size. The round nosed slug and the wall slug are observed in the channels. Correlations for predicting the fluidization regime transition in large fluidized beds are not adequate for predicting that in the mini- and microchannels. Also, differing from fluidization in a large bed, there is regime transition instability in that particulate fluidization is observed to form in the 700 mu m and 1 mm channels through the bubbling/slugging transition as the gas velocity increases beyond that for the fixed bed.