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Copper oxide (CuO) and cerium oxide (CeO₂) of various concentrations have been prepared through an ultrasonically assisted dispersion of CuO and CeO₂ nanoparticles (NPs) in which water and nanofluids (NFs) were formulated. The morphological properties of the CuO and CeO₂ NPs are reported. Few of the physicochemical properties that can influence the photocatalytic activities of the NFs are evaluated, such as viscosity, activation energy, density, thermal conductivity, electrical conductivity, alternating current (AC) conductivity, pH, stability, refractive index and optical band gap of the CuO and CeO₂ NFs. Viscosity studies have been made at four different temperatures (303 K, 308 K, 313 K and 318 K) and the activation energy is calculated and compared between the CuO and CeO₂ NFs. The thermal conductivity of the two NFs is calculated and compared. Electrical conductivity is measured for CuO and CeO₂ NFs using an impedance analyzer at different frequencies at 303 K. The dielectric constant and AC conductivity were studied. The electrical conductivity and pH of the prepared NFs are measured and the results are compared. The stability of the NFs is determined from Zeta potential values obtained from dynamic light scattering measurements. UV-Visible diffuse reflectance measurements were used to deduce the optical bandgap of the respective metal oxide NPs in the NFs. The photocatalytic efficiencies of the CuO NFs and CeO₂ NFs were evaluated using methylene blue (MB) as the model dye. The rate constant for the photodegradation of MB was higher for CuO NF as compared CeO₂ NF and also higher than simple NPs …