The influence of pre-treatment of a Ru/Al2O3 catalyst on the CO preferential oxidation activity was studied for the residential polymer electrolyte fuel cell (PEFC) cogeneration system. When a Ru/Al2O3 catalyst was heated in a H-2/N-2 mixed gas flow as the pre-treatment, the CO oxidation activity was highly improved at a low temperature range. When an activated Ru/Al2O3 catalyst was used with the pre-treatment, CO was removed from the simulated reformed gas to less than 6 ppm at [O-2]/[CO] = 1.0 and to less than 0.5 ppm at [O-2]/[CO] = 1.5. However, CO was not removed to below 10 ppm at [O-2]/[CO] = 1.5 on a non-activated Ru/Al2O3 catalyst without the pre-treatment. The temperature window, where CO was removed to below 10 ppm, of the activated Ru/Al2O3 catalyst was between ca. 90degreesC and ca. 140degreesC at [O-2]/[CO] = 1.0 and between ca. 85 degreesC and ca. 170 degreesC at [O-2]/[CO] = 1.5, while that of the non-activated Ru catalyst was between ca. 155 degreesC and ca. 180 degreesC at [O-2]/[CO] = 3.0. (C) 2003 Elsevier B.V. All rights reserved.