Na2CO3-impregnated activated carbon fibers (ACFs) have been developed as low-concentration H2S oxidation catalysts at ambient temperature. Two series of commercial pitch-based and poly(acrylonitrile)-based ACFs were used to evaluate the role of pore structure in the oxidation of H2S. The initial, impregnated, and exhausted materials were characterized using elemental analysis, N-2 adsorption, scanning electron microscopy (SEM), thermogravimetry analysis, and activity tests. The catalytic oxidation of H2S continued until all effective pores of the catalysts were blocked by the oxidation products. The saturation sulfur capacity was found to be in the range of 0.10-0.81 g of H2S/g of catalyst, with the value strongly dependent on the pore structure (especially the volume of pores larger than 0.7 nm) but independent of the nitrogen functional groups. Further quantitative analysis suggested that elemental sulfur as a dominant product mostly deposited in large pores (d > 0.7 nm), whereas sulfuric acid was preferably produced in small micropores (d < 0.7 nm). A possible mechanism of H2S oxidation with respect to the pore size of catalysts is proposed.