In this study, chromium (III) oxide (Cr2O3) and potassium oxide (K2O) nanoparticles (NPs) were synthesized by combustion. Mixed oxides of varying concentrations in the form of (1-x)Cr2O3/xK2O (x = 0.1, 0.3, 0.5, and 0.7) were prepared. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and optical measurements were used to investigate the properties of these nanoparticles. XRD indicates that the grain size decreased as the K2O content increased. Additionally, HR-TEM revealed several types of shapes, such as hexagonally-shaped nanoparticles. The optical measurements showed a blue shift indicating a minor modification to the band structure during the Cr2O3/xK2O mixing. The (1-x)Cr2O3/xK2O nanoparticles were used as catalysts in wastewater during a photocatalytic process. Orange G was chosen as an impurity in the water. The absorbance curves of Orange G in the water were measured at different times in the presence of the same quantity of Cr2O3/xK2O NPs. After 120 min, the efficiency reached 13% and 38% for (1-x)Cr2O3/xK2O NPs with x = 0.1 and 0.7, respectively. The experimental results were analyzed by pseudo first- and second-order models and intra-particle diffusion models. A first-order kinetic model had the best fit for the (1-x)Cr2O3/xK2O NPs. The strongest relationship occurred with a composition of 0.1Cr2O3/0.7K2O.