Since the total rate of conversion in a fluidized reactor depends on the transfer rate and the reaction rate in the continuous phase, the rate-determining step evidently depends on the relative magnitude of these rate coefficients. Measurement of the transfer coefficient should enable design calculations and evaluation of the optimum conditions to be made from fixed-bed studies. NOTATION a = surface area of particles per unit volume, sq.ft/cu.ft. A = a constant; area of the reactor, sq.ft. B = a constant C = concentration, moles/cu.ft. C,=specific heat of gas, B.t.u./ C, = specific heat of solid, B.t.