Synthesis gas containing predominantly H2 and CO is contacted with a suitable water-gas shift catalyst at a temperature 950°-1,200° F. and a minor portion of O2 for 0.01-2 min. The preferred pressure is atmospheric to 50-100 p. s. i. Usually, 0.01-0.25 mol. O2 per mol. of CO will be satisfactory. The quantity of O2 varies with the increase in the H2 : CO for a given temperature; the more O2 the higher the ratio. <1,500° F. the addition of O2 will increase the H2 : CO ratio. >1,500° F. the H2 : CO ratio will be decreased. 2 United States patents cited.