Carbon monoxide is a poison to the Pt anode in proton exchange membrane fuel cell (PEMFC). Preferential oxidation (PROX) is an effective method to reduce CO in hydrogen-rich gas streams to a tolerant level. In the present work, the effect of adding cobalt to Pt/gamma-Al2O3 on the PROX of CO was investigated. Our results showed that the addition of Co to Pt/gamma-Al2O3 could not only improve the low-temperature activity but also reduce significantly the loading of Pt in the catalysts. Over the catalyst 3% Co/1%Pt/ gamma-Al2O3 the conversion of CO was close to 100% at 90 degreesC and space velocity of 8000 mL g(-1) h(-1). In addition, the Co-promoted Pt/ gamma-Al2O3 catalyst showed good resistance to H2O and CO2 and could be operated in a wide range of space velocity. At temperatures above 90 degreesC, the existence of H2O in the feed increased the conversion and broadened the operating temperature range without worsening the selectivity. When space velocity was changed from 8000 to 80,000 mL g(-1) h(-1) and temperatures was kept between 120 and 160 degreesC, the conversion of CO was always over 99% and the decrease in O-2 selectivity did not exceed 10%. Furthermore, a strong opposite effect of the ratio of O-2 to CO on the conversion of CO and the selectivity of O-2 was observed. However, at the O-2/ CO ratio of 1.0 and temperatures between 120 and 160 degreesC, a satisfied balance between conversion and selectivity could be obtained.