New analytical solutions of concentration time curves are derived for an isothermal inert‐core spherical catalyst based on the mathematical models by taking into account first‐order irreversible reaction and mass transfer resistances in batch and fixed bed reactors. The effects of mass transfer resistances and Thiele modulus on catalytic efficiency and conversion of reactant are examined over a wide range of parameters for an inert‐core catalyst. The results show that the inert‐core catalyst can significantly improve the efficiency for fast catalytic reactions, where mass transfer limitations occur; however, the reactant conversion will decrease due to a lower loading ratio of active species of catalyst for inert‐core catalyst.