Experimentally, the formation of undesired heavy polycyclic aromatic hydrocarbons (PAHs) and chlorinated phenol byproducts in the supercritical water oxidation (SCWO) of 2-chlorophenol (2CP) catalyzed by CuO/ ZSM-48 at 673 K was significantly reduced. The reduction was attributed to the byproduct shape selectivity of the confined ZSM-48 channels. Speciation of active copper species in ZSM-48 was studied by X-ray absorption near edge structural (XANES) and extended X-ray absorption fine structural (EXAFS) spectroscopies, which suggested the existence of (Cu3O2)(n) (n(max) = 150, cross section of about 1.9 Angstrom) clusters in the channels of ZSM-48 (5.3-5.8 Angstrom). The diffusion coefficients of the copper-oxide cluster (determined by low-temperature electron paramagnetic resonance (EPR) spectroscopy) were greater than that of the tight-fit 2CP in the channels of ZSM-48 by at least three orders. Since the catalytic SCWO of 2CP was extremely fast, we consider the possibility that copper-oxide clusters were relatively mobile if compared with the 2CP in the confined channels of ZSM-48.