The 2,4-Dichlorophenol (2,4-DCP) solution (400 mg L¡Ð1) was treated by a wet air oxidation (WAO) process at temperatures of 393¡Ð453 K under a total pressure of 3.0 MPa using either Mn /£^-Al2O3 or Mn-Ce /£^-Al2O3 composite oxide as a catalyst. A COD reduction was found only 19.2% within 60 min if the WAO process was performed in a semi-batch type reactor without any catalyst addition; however, a higher COD removal of 69.4% or 71.4% was achieved when the Mn /£^-Al2O3 or Mn-Ce /£^-Al2O3 oxide was applied in the WAO process, respectively. A catalytic wet air oxidation (CWAO) of 2,4-DCP solution using the Mn-Ce /£^-Al2O3 oxide as a catalyst was conducted in another up-flowing fixed-bed reactor at 433 K under a pressure of 3.0 MPa in a space velocity of 4.0 hr¡Ð1. The COD reduction of the solution of 2,4-DCP was found above 61.4%. Also, both BOD5 and COD values in the effluent from the CWAO process was examined, and the BOD5/COD ratio was about 0.64. On the other hand, it is possible to treat the 2,4-DCP solution (¡Õ400 mg L¡Ð1) to meet the discharging regulation standard (COD¡Õ100 mg L¡Ð1) began in 1998 at Taiwan using a CWAO run and followed by an activated sludge treatment unit. The Ea values of the CWAO of 2,4-DCP using the Mn /£^-Al2O3 oxide as a catalyst were 20.77 KJ mol¡Ð1 and 23.99 KJ mol¡Ð1, respectively, for the first-stage and the second-stage reaction, respectively. In addition, the Ea values of the CWAO of 2,4-DCP over the Mn-Ce /£^-Al2O3 oxide were 14.77 KJ mol¡Ð1 and 23.30 KJ mol¡Ð1, respectively, for the first-stage and the second-stage reaction. Obviously, the Mn-Ce /£^-Al2O3 oxide does a better job in reducing the activation energy of the CWAO of 2,4-DCP than the Mn /£^-Al2O3 oxide does. Unfortunately, 2,4-DCP is hardly decomposed to become a low molecule weight carbon acids by the WAO run undergoing at 493 K without any catalyst addition. Several intermediates, such as 2-chlorophenol, phenol, catechol, oxalic acid, and formic acid, of the effluent from the CWAO of 2,4-DCP run over the Mn-Ce /£^-Al2O3 oxide were determined with a high-performance liquid chromatography.