Cyclohexanone was employed as a coreactant in the liquid-phase oxidation of cyclohexane using glacial acetic acid as a solvent and CoAPO-5 as a catalyst at 105-135 degrees C and 10 kg/cm(2). Addition of cyclohexanone increases the rate of cyclohexane oxidation and selectivity to adipic acid. When the cyclohexanone concentration increases, the rate will increase at 115 degrees C but decrease at 135 degrees C, while the cyclohexanone concentration has no obvious effect on the rate at 125 degrees C. The apparent activation energy for cyclohexane oxidation in the cooxidation system is lower than that of cyclohexane oxidation in the system without the addition of cyclohexanone. An increase of catalyst loading in the cooxidation system, though, increases the rate of cyclohexane oxidation but decreases the yield of adipic acid. Use of carboxylic acid, except formic acid, as the solvent is necessary for the oxidation of cyclohexane. Of acetic acid, propionic acid, butyric acid and n-pentanic acid, the use of propionic acid results in the highest reaction rate when the temperature is below 135 degrees C, while these four carboxylic acids give almost the same conversion after 2 h at 135 degrees C.