The homogeneous catalytic oxidation of ammonium sulfite in a microporous tube-in-tube microchannel reactor (MTMCR) was studied in this paper. It was found that the oxidation rate of ammonium sulfite in the MTMCR is 360 times higher than that in a packing column and could be further boosted about 4-6 times by the Co2+ catalyst. The effects of different operating parameters on the oxidation of ammonium sulfite were investigated. Experimental results indicated that the oxidation ratio of ammonium sulfite increased with the increase of the oxygen volumetric flow rate, but decreased with the increase of the liquid volumetric flow rate and sulfite concentration, and a smaller micropore size and annular channel width resulted in a higher oxidation ratio of ammonium sulfite. The oxidation ratio reached 80% under the optimum operating conditions in the catalytic oxidation process. This study reveals that the MTMCR is a potential intensification device for the oxidation process of ammonium sulfite. (C) 2014 Elsevier B.V. All rights reserved.