A rigorous multicomponent multi gas-solid reaction model is developed. It is based on the particle - pellet model and considers the transient nature of the system, inter- and intra-particle heat and mass transfer and the variation of structural parameters with reaction. The model equations describing the structural changes along with property evaluation schemes are formulated for a general multicomponent system. Simulation results are compared with experimenter data for carbon gasification and the reduction of a nickel oxide / hematite mixture. The match between model and experiment was found satisfactory. Both concentration and temperature profiles and other parameters are examined.