In gas-solid reactors, particularly circulating fluidized beds (CFB) it is becoming increasingly m ore important to be able to predict the conversion and yield of reactant species given the ever rising cost of the reactants and the ever decreasing acceptable level of effluent contaminants. As such, the development and use of predictive models for the reactors is necessary for most processes today. These models all take into account, in some manner, the interphase mass transfer. The model developer, unless equipped with specific experimentally based empirical correlations for the reactor system under consideration. is required to go to the open literature to obtain correlations for the mass transfer coefficient between the solid and gas phases. This is a difficult task at present, since these literature values differ by up to 7 orders of magnitude. The wide variation in the prediction of mass transfer coefficients in the existing literature is credited to flow regime differences that can be identified in the cited literature upon careful inspection. A new theory is developed herein that takes into account the local hydrodynamics. The resulting model is compared with data generated in the NETL cold flow test facility and with Values from the literature. The new theory and the experimental data agree quite well, providing a fundamentally based mass transfer model for predictive reactor simulation codes. Published by Elsevier B.V.