A series of tetragonal zirconia-supported CuO oxide catalysts with various CuO loadings were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). electron spin resonance (ESR), ultraviolet and visible diffuse reflectance spectroscopy (UV/vis-DRS), and temperature-programmed reduction (TPR) measurements. The results indicate that the dispersion capacity of copper oxide on this support is approximately 8.6 Cu2+ ions/nm(2) ZrO2. The state of the resulting supported copper species depends on the CuO loading. At CuO loadings below the dispersion capacity, only highly dispersed copper ion species are, present on the surface of t-ZrO2. In particular, isolated Cu ions are the predominant species at low loadings. In contrast, pair Cu ions become the most abundant species at loadings near the dispersion capacity. It has been proposed that these dispersed CuO (isolated and paired Cu ions) have a symmetric 5-fold-oxygen-coordination symmetry (C-3v symmetry) and can be described as distorted octahedra with a missing corner or a trigonal bipyramids. Finally, at CuO loadings above the dispersion capacity the formation of crystalline CuO is observed. TPR results reveal that the dispersed Cu ion species have a different reducibility from CuO crystallites. presumably due to strong interactions between these species and the t-ZrO2 support. The catalytic activity of these CuO/t-ZrO2 catalysts for the decomposition of NO can also be directly correlated to CuO dispersion, with paired Cu ions being the most active species for this reaction.