Copper (X-2-salen) complexes, where salen(2-)=N,N＇-ethylenebis(salicylideneaminato) and X=H, Cl, Br or (NO2), encapsulated in the cavities of zeolites NaX and NaY, have been synthesized and characterized by various physicochemical measurements. Samples obtained by synthesizing the complexes in situ in the cavities of the zeolite by the ＇flexible ligand＇ method contain a higher concentration of the complex than those obtained by synthesizing the zeolites around the pre-formed copper complex. Substitution of the aromatic hydrogen atoms of the salen ligand by electron withdrawing groups like -Cl, -Br and -NO2 has two major effects : (1) retention and concentration of the copper complex in the zeolite cavities is enhanced (due to the larger size of the substituents) and (2) the electronic and spectral properties of the encapsulated complex are modified. The rates of decomposition of H2O2 over these encapsulated, substituted copper salens approach those of natural catalase enzymes and correlate with the rates of oxidation of phenol.