The aerobic liquid-phase oxidation of p-xylene (PX) over metalloporphyrin and Co(OAc)(2) was studied, and the co-catalysis between metalloporphyrin and Co(OAc)(2) for the oxidation of PX was discovered for the first time. The results showed that both the PX conversion and terephthalic acid yield could be increased significantly despite the fact that only a minute amount of metalloporphyrin was added to the reaction mixture in addition to the Co(OAc)(2) catalyst. The effects of the structure of metalloporphyrin and reaction conditions such as temperature, air pressure and catalyst concentration on the overall reaction performance were also studied. A possible mechanism for the observed synergy between metalloporphyrin and Co(OAc)(2) as co-catalysts for the aerobic liquid-phase oxidation of PX was proposed based on some experimental observations. The results suggested that the PX oxidation was improved because of the acceleration of the chain initiation of PX oxidation by metalloporphyrin, and the acceleration of the chain initiation itself was due to the ease of peroxide formation over metalloporphyrin. (c) 2008 Elsevier B.V. All rights reserved.