Simplified expressions to calculate the extent of reaction in a fluid bed show good agreement with the experimental data of Lewis et al. (1959) and Swaay and Zuiderweg (1972). The contribution of the dilute phase seems more important than that of the jetting zone or reaction factor (Hatta number) in usual fluid bed operations. The effect of the dilute phase is discussed. It is profitable to use the dilute phase for the consecutive isothermal and endothermic reaction, but unprofitable for the exothermic reaction. These characteristics depend upon the nature of reactions and careful considerations as to the choice of the gas velocity, particle characteristics, and reactor design will be necessary.