The ability of Zeolites molecules to selectively catalyze the reactant(s) is called shape-selectivity of Zeolites. The ability of selectively catalyzing the reactants is achieved through their pore structure. The size of the pores in Zeolites varies from 260 to 740 pm. Thus, the molecules which are small enough to enter these pores are absorbed by Zeolites. Catalysis of only adsorbed molecules takes place. Depending on the size of reactant and product molecules compared to size of pores, reaction proceeds in a specific manner. Therefore, Zeolites perform selective-catalysis due to their unique pore structure or shape.