Herein, the formation of carbon monoxide as a harmful product upon the Catalytic Wet Peroxide Oxidation process is studied in presence of different solid catalysts: an iron supported activated carbon catalyst, a metal free catalyst based on Graphene Nanoplatelets, and 1.6 wt.% Fe containing Cr2AlC MAX phase catalyst. The CWPO performance and the evolution of the gas effluent have been compared to that obtained in a conventional Fenton process. Carbon monoxide yield released was significantly lower in Catalytic Wet Peroxide Oxidation process in relation to that obtained in the Fenton process, where CO concentration reaches a maximum of 6651 mg/Nm(3). By contrast, in presence of activated carbon-Fe catalyst and, notably, Graphene Nanoplatelets and Cr2AlC MAX phase catalysts, a more progressive phenol and aromatics intermediates oxidation resulted in a much lower CO maximum concentration in the gas phase at the exit of the reactor of 2454 mg/Nm(3), 170 mg/Nm(3) and 187 mg/ Nm(3), respectively. Hence, when compared to the homogeneous Fenton oxidation, Catalytic Wet Peroxide Oxidation process results be a more sustainable treatment for high-loaded phenolic wastewaters by decreasing the hazardous CO gaseous emissions avoiding this way a secondary pollution during the oxidation process.