The stabilization of a mononuclear copper(II) complex with one Mlm(2)Pr ligand [Mlm(2)Pr = 3,3-bis(1-methylimidazol-2-yl)propionate] in the supercages of zeolite Y was attempted, and the resulting materials were tested for their activity in oxidation catalysis. The preparation procedure yielded initially two species (labeled 1 and 2) within the pore system of the zeolite material, which differ in molecular structure and chemical composition as determined by UV/vis, ESR, IR, and XAFS spectroscopy. In species 1, the copper was found to be five-coordinated, with one Mlm(2)Pr ligand in a facial-type NNO coordination toward copper, the other two coordination sites being occupied by oxygen atoms from either the zeolite framework and/or a water molecule. The total charge of this complex is 1 +. In species 2, the copper is surrounded by two Mlm(2)Pr ligands, both in a facial-type coordination mode, identical to the homogeneous Cu(Mlm(2)Pr)2 complex. This neutral species 2 is easily washed out of the zeolite, whereas the mononuclear species 1 remains inside the zeolite material upon washing. The spectroscopic characteristics and activity for 3,5-di-tert-butylcatechol and benzyl alcohol oxidation of species 1 compared closely with that of the zeolite-immobilized Cu(histidine) complexes but differed from that of the homogeneous Cu(Mlm(2)Pr)(2) complex. It was therefore found that encapsulation in zeolite offers a route to stabilize a 5-fold-coordinated copper complex with novel catalytic properties. This 1:1 Cu(Mlm(2)Pr) complex is not formed in solution.