ABSTRACT Riser reactors are widely used for catalytic cracking, combustion of coal, calcinations of lime etc. The reactions are fast and mass transfer limitation can play an important role. Information on gas to particle mass transfer in high velocity fluidized beds is very limited. Available experimental observations on gas to particle mass transfer in high velocity fluidized beds are reviewed. Gas to particle mass transfer coefficient increased with increasing gas velocity, decreased with increasing solid circulation rate. and decreased with increasing particle size. A model is developed considering that particles move as clusters. The model compares well with the available results.