To maximize the adsorption of COS (carbonyl sulfide) gas, activated carbon (AC) with a large surface area was modified with K as a low-cost adsorbent capable of bonding at low temperatures with the S in COS. The contents of AC and K were fixed at 80% and 20%, respectively. The COS gas was dynamically adsorbed by AC-K at an optimum temperature of 210 degrees C with a sulfur-adsorption capacity of 56.1 mg/g. The structure of the K adsorbent was confirmed as K2CO3 via X-ray diffraction and the K components were highly dispersed on the surface of the AC, as indicated by SEM and EDAX analyses. Furthermore, the correlation between the specific surface area of AC-K and COS adsorption was examined using FT-IR, XPS, and MASS analyses. The mechanisms by which the COS adsorbed/desorbed from the adsorbent were determined and an overall mechanism describing COS adsorption behavior was proposed.