<p>The use of photocatalysts for water purification is of great interest. The photocatalysts use solar energy to degrade organic pollutants. In this work, Cd-Mn–O nanocomposites (CMO NCs) were synthesized through simple co-precipitation and hydrothermal routes. The co-precipitation synthesis was fabricated without and also with capping agents, CTAB, PEG4000 and PVA. The hydrothermal synthesis was promoted without surfactant at 120, 150 and 180&#xa0;°C for the different times. We also investigated effects some parameters, including, the surfactant, temperature and time in this synthesis. SEM images showed that multifaceted structures with diameters 10–300&#xa0;nm, and clumps of the semi-spherical and multifaceted structures with different sizes (50–400&#xa0;nm) were formed via co-precipitation and hydrothermal methods, respectively. With the hydrothermal route, nanorods and nanowires with diameters less than 50&#xa0;nm could also be seen among these structures. TEM images confirmed SEM results and showed the formation of the spherical and multifaceted structures with diameters ranging from 2 to 30&#xa0;nm, for the product prepared via co-precipitation. XRD patterns showed the formation of CdMn₂O₄ and CdO phases. The peaks at 500–700&#xa0;cm⁻¹ in FT-IR were related to the vibrations of Mn‒O and Cd‒O bonds. The presence of Cd, Mn, and O peaks in EDS confirmed the formation of CdMn₂O₄. Also, we highlighted the photocatalytic activities of CMO NCs, for the first time. To optimize the photocatalytic activities, we utilized both cationic and anionic dyes, including Rhodamine B (RB), Malachite green (MG), and Eriochrome Black T (EBT). We investigated the effects of some parameter to assess the UV-assisted photocatalytic activity of the CMO NCs. Maximum decolorization was achieved for the degradation of 5&#xa0;ppm of EBT, with 0.05&#xa0;g photocatalyst dosage, under UV irradiation after 120&#xa0;min. It was selected as an optimum condition. The optimum CMO NC could catalyze the degradation of EBT by about 95.5%.</p>