Abtraet-A general model describing simultaneous independent reactions between one fluid reactant and two solid reactants is presented. Such reaction systems are attracting much attention in the areas of oil shale retorting, reduction of mixed metal oxides with a reducing gas, leaching of mixed minerals, and roasting of mixed sulfides. The model has been formulated in general terms so as to allow the incorporation of specific details of an actual system. The law of additive reaction times previously developed for single fluid-solid reactions and found useful for the analysis of the reaction between one solid and two fluid reactants has been applied to the reaction system under consideration. It has yielded a useful approximate solution, which obviates the necessity for repeated numerical solution of a second-order differential equation with split boundary conditions and removes the possibility of encountering the numerical stiffness problem. INTRODU~ION Many industrial processes involve simultaneous fluid-solid reactions between one fluid and two solid reactants. In the combustion retorting of oil shale, the reaction of oxygen with char which contains carbon and hydrogen provides an example. In this and also in the burning-off of coke from coked catalysts, the oxidation of hydrogen in the coke occurs more rapidly than that of carbon. Another example is the oxydesulfurization of coal in which the inorganic sulfur is oxidized more rapidly than the organic sulfur. Other examples include the sulfation of dolomite, the gaseous reduction or chlorination of mixed metal oxides such as the reduction of nickel laterite or the chlorination of ilmenite, the leaching of mixed sulfide or oxide minerals, and the roasting of mixed sulfide ores. In a previous article[l], we presented the analysis of a reaction between one solid and two fluid reactants and showed how the law of additive reaction times, proposed previously for a single fluid-solid reaction[2], provided useful approximate solutions for that system. Other previous work in the area of simultaneous fluid-solid reactions was also presented[l]. Most previous work on the reaction between one gas and two solids involves the study of the system in which one reaction is fast and the other slow, or both reactions are very fast or very slow. However, a more general description of the reaction *Author to whom correspondence should be addressed.