Abstract: Pseudo solid-solid reactions,which proceed through gaseous intermediates are of considerable im-portance in metallurgy,such as the reduction of metal oxides with carbon and the segregation roastingof oxidic copper or niche ores.A model for paired processes has been derived for describing in detail the reduction of ferrousoxide with carbon in various rate-controlling regimes.This model is characterized by two parameters,k_1’and k_2’which include the intrinsic chemical reaction rate constants,k_1 and k_2,the diffusion coefficientD~A_c1 and the mass transfer coefficients,h~A_m1 and h~C_2:Analytic solutions have been obtained for eight extreme cases,and a series of numerical solutiousobtained with the aid of an electronic computer are presented.A calculated example based on expcri-nental data is included at the end of the paper.