A method for separating oxidation catalysts used for producing trimellitic acid, which comprises the steps of: adding water to a reactor effluent in a volume ratio of water to the reactor effluent of 0:1 to 11:1, the reactor effluent being in a slurry state and resulting from oxidizing pseudocumene with air in an acetic acid and in the presence of cobalt, manganese and bromine catalysts; heating the mixture of water and the reactor effluent to a temperature of 25.degree. to 140.degree. C. to make the slurry be a diluted reactor effluent in a liquid phase; passing the diluted reactor effluent through ion exchange resins, to adsorb cobalt, manganese and bromine ions on the resins; regenerating the resins with an eluting solution; and isolating the oxidation catalysts from an effluent, shows recoveries of at least 99 wt % for cobalt, at least 99 wt % for manganese and at least 90 wt % for bromine.