Abstract: A reduced graphene oxide (RGO)-supported CoMn₂O_3.5 composite (CoMn₂O_3.5-RGO) was synthesized by a coprecipitation method in a GO suspension using Co(NO₃)₂·6H₂O and MnSO₄·H₂O as Co and Mn sources, respectively, followed by heat treatment at 600℃ for 0.5 h. Results indicate that CoMn₂O_3.5 particles with an average diameter of about 300 nm decorated the RGO sheets and self-organized into microcubes. The CoMn₂O_3.5-RGO as a Fenton-like catalyst exhibited the high activities for the decomposition of various dyes including methylene blue (MB), rhodamine B (RhB) and golden orange II (OGII). 10 mL solutions of the OGII, MB and RhB dyes (50 mg L^-1) are 100% decolorized with 5 mg of the CoMn₂O_3.5-RGO in 50, 70 and 80 min, respectively. The high activity of the catalyst is closely related to a synergistic effect of RGO and CoMn₂O_3.5, where the dyes are adsorbed on the surface of RGO by-interaction and retained in close proximity to the active sites of CoMn₂O_3.5.